What metrics matter, and what is “good performance”?

One of the things that I found that gets glossed over a lot in the existing performance advice out there is explaining what metrics matter, and what qualifies as good when it comes to performance

The first question is a lot easier to answer than the second.

What metrics matter?

There are three main metrics that I’m currently focusing on in my work, and for most projects I think these will apply:

  • Download Size – how much data is physically downloaded to you user your website? This one is pretty easy to wrap your head around – the more data a user needs to download, the slower your site will be. You can never be too rich, too thin, or have to small a download size.
  • Time to download – how long does a typical user take to download the resources needed for your site?
  • Number of requests – how many separate requests are made in order to load your site. Each request for an image, css file, etc, matter

For the main project that I work on, I primarily focus on these. I personally feel that focusing on these provide the most bang-for-the-buck when it comes to devoting developer time to performance.

You’ll find a lot of advice about how to fine tune rendering, avoiding repaints, etc, but all of that won’t matter much if you haven’t figured out these three things first.

Which one metric matters most?

It may seem obvious, but download size. This supersedes all other performance issues. It is the easiest to get wrong, and often is the easiest to fix.

Download size matters most because it is what you are pushing that actually costs your user money. Data plans, especially not in the US, charge by the megabyte. There is a [great resource that will show you how much a given URL will cost a user]( GET LINK) to visit based on data plans around the world.

What is “good” when it comes to performance?

This is a much harder question to answer. As with all difficult questions, the answer is “it depends”.

This is a reason why it’s good to have a clear understanding of the business goals of your site, and the metrics by which success is measured.

What constitutes good performance for a large media site is very different from that of say, a blog.

There are general guidelines floated out there:

  • Under 1MB download size
  • Under 1 second until the site seems usable to the user

These are good goals to shoot for, but also take them with a huge grain of salt.

At my current position, our most popular type of page involves displaying videos – there is no way we’ll every be close to 1MB of download size.

Better performance is always better

The way I look at it, continually trying to improve performance is the right mindset. It’s an ongoing journey, which, really, will never be “done”.

This also takes constant vigilance and monitoring of your site performance metrics. How one does that will be the subject of the next couple posts.