Measuring Website Performance

Speed metrics that matter, and how to measure them

Website performance is important for several reasons. Being able to measure, collect, and review meaningful performance metrics is the first step in verifying that your website is performing as expected, and can help you figure out if you are providing a good user experience to your site visitors.

Before getting into the details about how and what to measure, make sure that you have familiarized yourself with the basics of website performance.

Performance Data Sources

Before talking about the different types of performance and UX metrics that can be measured and collected for websites, it's important to differentiate between the various sources of performance data.

We will group these into two categories:

Synthetic Tests / Lab Data
- Test results from synthetic testing tools such as PageSpeed Insights, WebPageTest, Pingdom, GT Metrix, New Relic Synthetics, etc.
Real User Monitoring / Field Data
- Data that is collected from actual visitors to your site, via Chrome's anonymized reporting to the Chrome User Experience Report, or from Real User Monitoring (RUM) code inserted into web pages.

Which one(s) should you use? We answer that in a bit, after we talk about the types of performance metrics.

Types of Performance Metrics

There a many, many metrics and attributes relating to page loading and website performance that can be measured for sites. Here's a partial list:

DNS Lookup time	TCP Connection time	TLS Handshake	Server Response time	Time to first byte (TTFB)	Download time
Network connection speed	Time to last byte	Start Render time	Page Load Event	Speed Index	First Input Delay
Total Blocking Time	Number of Requests	Largest Contentful Paint	DOM Content Loaded	First Contentful Paint	Cumulative Layout Shift
Bytes Downloaded	Browser cache hit %	DOM Interactive	First Paint

But which ones are important?

If we focus our concern on actual human visitors and user experience, we can distill this list down quite a bit.

Google has defined a list of web vitals on their web.dev site, including a set of Core Web Vitals.

We will cover web vitals and Core Web Vitals in more detail in another post. But next, let's continue our discussion regarding lab data vs. field data.

Lab vs. Field Data

Google's web.dev site has a page about this called Getting started with measuring Web Vitals.

As we discussed in our post covering website performance basics, you almost always want to focus on performance and speed as it relates to actual site visitors.

In this regard, field data, or metrics collected from real users, should be used to help you understand what kind of performance actual visitors to your site are likely experiencing.

Synthetic testing tool results should never be used as goal or benchmark themselves.

Starting with a tool like GTMetrix, and making the scores better, then considering yourself "Done"
=
Declaring premature victory, while worshiping a false God

Testing tools are meant to be used to understand page performance for a specific number of page loads, under very specific conditions.

Before you can use the tools effectively, you must first understand the basics of website performance thoroughly.

Coming next ...

CrUX report, PageSpeed Insights, SiteDistrict Web Vitals dashboard, and more ...