How to test the speed of a website

Why test site speed?

When automobile manufacturers develop a new model of a car, it may accelerate quickly and drive smoothly on paper, but the manufacturer can't know how well the car actually runs until a test driver takes it out on the track. Similarly, how a site performs in a local testing environment is not always a good indication of how it will perform in the wider Internet, which spans across a variety of network conditions and in various locations.

Website speed tests aim to simulate real-world conditions and provide data on how well a website actually performs. A website speed test should let developers know not just how fast their site or application is, but also which elements on the page are causing slowdowns.

Why is site speed important?

Websites that perform poorly can frustrate users, driving them away. Slow site performance negatively impacts search rankings (or SEO), conversion rates, and the overall user experience.

How can developers test the speed of their websites?

A number of companies and organizations offer website speed tests. Many speed tests are able to identify individual elements of a webpage that are slowing the page down, in addition to providing performance metrics. Cloudflare offers free CDN services for boosting website performance and reducing latency.

A number of online providers also offer website speed tests, and typically all that's necessary to start the test is to paste the URL into the provided field.

What performance metrics will a site speed test provide?

The basic Cloudflare speed test measures the following:

• Load time: The time it takes for a web browser to finish downloading and displaying the webpage (measured in milliseconds)
• Time to First Byte (TTFB): How long it takes for the browser to receive the first byte of data from the web server (measured in milliseconds).
• Requests: The number of HTTP requests for resources that a browser has to make in order to fully load the page.

Not all speed test providers will break down site speed using the same performance metrics. Other performance metrics include:

• DOMContentLoaded (DCL): This measures the time it takes for the full HTML code of the page to be loaded; images, CSS files, and other assets don't have to be loaded.
• Time to above-the-fold load: 'Above the fold' refers to the area of a webpage that fits in a browser window without a user having to scroll down.
• First Contentful Paint (FCP): The time at which content first begins to be 'painted,' or rendered, by the browser. This can be any aspect of the page, including text, images, or non-white background colors.
• Page size: The total file size of all content and assets that appear on the page.
• Round trips: This metric counts the number of round trips necessary to load the webpage. When an HTTP request travels all the way from a browser to the origin server, and the server's HTTP response goes all the way back, this constitutes a round trip.
• Render-blocking round trips: A subcategory of round trips. 'Render blocking' refers to resources that have to be loaded before anything else can be loaded.
• Round trip time (RTT): The amount of time the round trips take.
• Render-blocking resources: Certain resources, like CSS files, block other parts of the page from being loaded if they are not yet loaded. The more render-blocking resources a webpage has, the more chances there are for the browser to fail to load the page.