The ultimate beginners guide to web performance and speed

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Above is a video version of this guide. It goes in-depth into many of the topics, and includes a few more. The presentation was given to a group of web developers at The Church of Jesus Christ of Latter-day Saints, as is slightly tailored to some of their web properties. Enjoy!

This guide is loosely arranged in order by bang-to-buck ratio, so simpler optimizations are listed first (that generally have a immediate impact).

  1. Optimize images
  2. Don’t use images
  3. Lazy load images
  4. CSS at the top, Javascript at the bottom
  5. Load scripts asynchronously
  6. Ditch the library
  7. Minify and Concatenate
  8. Take advantage of browser caching
  9. Enable compression
  10. Take advantage of server caching
  11. Optimize database queries
  12. Separate static assets onto a different domain
  13. Use a content delivery network
  14. Flush partial responses

1. Optimize images

It’s not uncommon for an image straight from photoshop or a digital camera for weigh several megabytes. Large and unoptimized images can be the number 1 cause of slow loading pages. Luckily, this may be the simplest optimization as well.

This vacation image (right from my digital camera) is 12.8m. It is also 4000×6000 — which is far too large for a webpage. Reducing the pixel size in photoshop to 1000×667 takes the file size down to 480.7kb. This is still far too large. In the Photoshop Save For Web menu, we can usually reduce the jpeg quality to between 60%-70% without noticing any artifacting or degradation.  My rule of thumb is to move the quality sider down until I notice a difference, then take it up by a third. I’ve reduced the jpeg quality on this image to 62% and the new file size in 145.9 kb. This is still large for the web, but more acceptable for a photograph.

example photo for optimization

Optimized vacation photo

We can further reduce the size by using other image optimization tools, such as ImageOptim. This tool reduces image file size by making imperceptible quality adjustments, as well as by removing EXIF and other metadata from the file.

optimizing images with imageoptim software

ImageOptim reducing image file size

2. Don’t use images

Images are only really needed for displaying photos, screenshots, or other graphics. With the advance of CSS3 support, there are almost no situations were image files are necessary for page layout or design.  border-radius and   linear-gradient alone have reduced the need for most of our image dependancies.

Icons were another common image use-case. We can now replace these with the more semantic use of icon fonts or svg “sprites.” Icomoon is an excellent tool for creating both icon fonts and svn icons. Chris Coyer puts icon fonts and svgs in a informative cage-match that excellently lays-out the pros and cons of both methods

Finally, the One Less JPG movement reminds us that “before you go worrying about how to minify every last library or shave tests out of Modernizr, try and see if you can remove just one photo from your design. It will make a bigger difference.”

3. Lazy-load images

Inline images

If an image falls below the viewport (not immediately visible), load that image “lazily” — or after the main content has loaded. We can accomplish this simply and semantically, and in a way that still works for non-javascript users.

There are many expertly programmed lazy-load images scripts, so I’ll just outline the logic here:

CSS-tricks has an modern lazy-load script which seems to work well.

Background images

Lazy loading background images in CSS uses the same logic as inline images, with a slightly modified implementation:

4. Css at the top, Javascript at the bottom

The order and placement of CSS files and Scripts is important. CSS should be placed in the head element. As soon as the styles are downloaded and parsed, they can be applied to their DOM element matches — increasing perceived load time.

Included Javascript files will block the parsing and execution of a page, and should be placed at the bottom of a document to avoid the block.

5. Load scripts asynchronously

If a page is built with progressive-enhancement in mind, most scripts should be loaded asynchronously, or after the page itself. These scripts will enhance the experience of the users, and won’t be missed for the split-second before they are loaded.

Most popular libraries (read: jQuery) have built-in asynchronous script loading, but it is also simple to implement without a library. HTML5Rocks has a deep dive into async script loading. It’s long, but well worth the read.

6. Ditch the library

Speaking of popular front-end libraries: do you really need jQuery? The tongue-in-cheek site, Vanilla-JS offers up native javascript solutions for common jQuery uses. Indeed, many of the uses of jQuery can be reproduced by simple native JavaScript:

Your code can get a little more verbose when iterating over a set of elements:

 Case-by-case

Ditching the library isn’t a hard-and-fast rule. There are many things that jQuery provides that are difficult to achieve with out.

For example, jQuery’s event binding interface can be overloaded to provide event delegation — binding an event handler to an element’s parent in order to allow dynamic loading or modification of new child elements without losing the event handler.

Here is a native event delegation function I wrote to add this functionality natively:

As you can see, much of the native implementation deals with the inconsistencies in browser implementation. jQuery and other libraries “pave over” these differences, and provide a consistent API for developers. Additionally, library implementation code has been tested by many users over many browsers. Library usage benefits from thousands of hours of bug and edge-case fixes.

When deciding whether or not to ditch the library, you need to carefully assess the needs of your site. Are there complex requirements or compatibility needs?  Do you have time to write and test native implementation? There are certainly use cases where a library is not needed, but consider carefully before you make that decision.

7. Minify and Concatenate

If your CSS or JavaScript files are being served to a user in the same form that you authored them, then there is a performance penalty to pay for downloading all the unnecessary whitespace and descriptive naming conventions that make for good development.

Minification is the process of compressing files, removing unnecessary whitespace, line-breaks, and comments. It is often coupled with Uglification, which transforms code by renaming variables, and refactoring functions into the most compressed format. Any CSS or JS that makes its way to an end user should be both minified and uglified.

Concatenation is the process of combining multiple files into one. This reduces the number of requests, and is especially useful when optimizing for mobile networks where latency is often the bottle-neck.

There are many good stand-alone minification and concatenation tools, but I recommend integrating both of these processes into your build-step, using Grunt, Gulp, or similar.

8. Take advantage of browser caching

Once a user has visited a page on your site, assets have been downloaded and can be cached on the users system. Typically, static assets are cached: CSS, JavaScript, Image files, etc, as well as dynamic requests: search results that change infrequently, lookups, or other ajax requests.

Asset caching is controlled via to headers sent with the file:  Cache-Control and ETag. These headers are sent by the server with the file response.

From Google’s Leverage Browser Caching:

  • Cache-Control  defines how, and for how long the individual response can be cached by the browser and other intermediate caches. To learn more, see caching with Cache-Control.
  • ETag  provides a revalidation token that is automatically sent by the browser to check if the resource has changed since the last time it was requested. To learn more, see validating cached responses with ETags.

There are different ways to set these headers depending on your website platform. This guide to caching headers does a good job of explaining the different options available for the header. Ultimately, you will need to use a method that is correct for you specific platform.

9. Enable compression

This is a simple optimization, and comes enabled on most servers by default. By enabling gzip compression, assets served are run through a compression algorithm and decompressed by the client. The details of how to enable gzip compression are different for each platform, so find a guild that is specific to your technology to install or enable it.

10. Take advantage of server caching

As dynamic pages are built, there can be many database requests for each page. Consider the typical blog: the article content, related stories, and comments are each separate database queries. The comments will be updated frequently, but the article itself and the related stories will not change once published. We can save the results of these queries in memory or on the disk for future use (saving a expensive database query). Like browser caching, the implementation will vary from platform to platform, but the theory is the same. Each platform will have it’s community recommended caching solution, and honestly, the bang-to-buck ratio of creating your own is slim.

The logic of server caching is generally:

This pseudocode saves the cached page to the file system, but you could just as easily store data in memory or in an object-store. Google “caching in [your system]”, or read up on Varnishmemcached, redis for more information.

11. Optimize database queries

Expensive database queries can add hundreds of milliseconds to the lifecycle of a request. Optimizing queries is the process of:

  1. Identifying expensive database queries
  2. Refactoring
  3. Repeat

Identifying expensive queries

In general, look for queries that ask for more then they need. This can take the form of  SELECT * , when all you actually need is  SELECT first_name . Any query that returns more than you need, JOINs with more than is needed, or contains correlated subqueries should be suspect. Every database system is different, so there is silver-bullet when it comes to identifying bad queries.

Refactoring

Once bad queries are identified, use the database tools to understand the workings of the query. Many databases support an EXPLAIN  keyword that returns information about how a query is interpreted and executed by the database engine.

With the help of EXPLAIN, you can see where you should add indexes to tables so that the statement executes faster by using indexes to find rows. You can also use EXPLAIN to check whether the optimizer joins the tables in an optimal order. – MySQL Documentation

12. Separate static assets onto a different domain

When a request is made to a server, the client machine will send data that is relevant to the request to help the server complete it. This way, when a user requests a new page, data, such as an session cookie, is sent to the server. Without this interaction, dynamic and personalized user experiences would not be possible. However, there are many types of files where cookies are not relevant. This is the case with static assets (CSS, JavaScript, images, etc). To make static asset request smaller, serve assets from a static-only domain. You may have noticed while loading a YouTube video, a network request to *.ytimg.com. ytimg.com is YouTube’s cookie-less image domain. That way, when any image is requested by YouTube (like a thumbnail) it doesn’t have to send or receive cookies for every request.

You don’t necessarily need to host your static files on a different server, or even in a different application. You can configure most servers to use a separate path and domain for a folder using the rewrite rules system for your particular platform.

13. Use a Content Delivery Network

Content Delivery Networks, or CDN’s, store contents of your website on servers that are more physically close to the end users location. When a request for an asset is made, the CDN will look on it’s servers to see if it has a cached copy. If so, it will return that to the client — avoiding any traffic to your servers completely. If the file is not available, the CDN will request the asset from the server, store it, and then send it to the user. Not only does this decrease load on your server, but it speeds up the request by delivering the asset from a physically nearby server.

Akamai, and AWS both have very popular commercial offerings. However, you don’t have to be Facebook-scale to use a CDN. One company, CloudFlare, offers free CDN access for websites that server under a certain amount of traffic. Cloudflare’s product setup is relatively simple: you edit your DNS to point to their nameservers, and all traffic is routed through their network. Assets are cached as they are requested, and served to users from their servers around the world.

14. Flush partial responses

Some background: during a request lifecycle, there are three main functions that take the majority of the time to build a page:

  1. Network transmission time & latency
  2. Building the page on the server
  3. Rendering the page on the client

Often, as each part of the cycle is busy working, the other two are idle. We can perform this optimization to utilize the other two, and reduce the total time of the request.

Consider a request for “page-2”:

  1. the request is sent from the client
  2. network time
  3. the server receives and routes the request
  4. page renderer is found
  5. database queries are made
  6. page is built
  7. page is sent to the client
  8. network time
  9. page is received by the client
  10. client renders the page
  11. client makes requests for linked assets

Now, during step 4-6, the network and client are not utilized. Flushing a partial response means starting to send data over the network to the client before the entire response is ready. Segmenting a response in this way “pipelines” the process and decreases the time a client waits to receive the first response packet.

Each platform is different, but this pseudocode shows the general theory:

Flushing the output of the page in this way allows the client to receive the header before the entire page has finished building, and it may start rendering immediately.

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