Measuring web page load times in JMeter

I want to measure the page load time of a web application with a maximum of 10 concurrent virtual users doing actions on different website pages. I am concerned that the web app pages rendering time will be considerably more than simply what is measured in JMeter.

I am thinking of making a JMeter script using:

• HTTP Request Defaults with the "Retrieve All Embedded Resources" checkbox as well as the parallel downloads checkbox - to fetch embedded resources like scripts, styles, etc. from the pages

• HTTP Cookie Manager - for representing browser cookies, enabling cookie-based authentication and maintaining sessions.

• HTTP Cache Manager - for simulating resources being returned from the browser's cache on subsequent requests of a same logged-in virtual user

• HTTP Header Manager - to represent browser headers like User-Agent, Content-Type, encoding, etc.

However, I think this is the closest I can get to the real page load time using purely JMeter by grouping requests under Transaction Controllers to determine a Page load time.

As I understand the problem, using only JMeter, a Page Load would be the sum of:

• Request Processing time + Latency (time taken by network while transferring data)

But, by simulating a browser, a page load would be a sum of:

• Processing time + Latency + Rendering time

Rendering time would include things such as:

• Client-side Javascript execution – AJAX, JS templates, etc.
• CSS transforms – 3D matrix transforms, animations, etc.
• 3rd party plugins – Facebook likes Double click ads, etc.

In order to get a feel of this rendering time, I would like to know if someone with more experience thinks it would be a good practice to configure only 9 concurrent users to make different scripted navigations and interactions with the website and then have another 1 virtual user in another Thread Group do scripted navigations but using the Selenium Webdriver plugin for JMeter.

Or, since it's a small number should I configure all users to run through the Selenium Webdriver plugin and spawn 10 browsers on the same load generator? (with this approach I am more concerned about UI tests flakiness or instability and with the load incurred on the load generator machine)

What do you think? Any opinion is greatly appreciated.

There are 2 separate actions:

1. Backend performance testing. Given you properly configure JMeter to behave like a real browser you can simulate its network footprint and conduct the realistic load pattern. Apart from configuration you applied already you might also want to simulate AJAX requests using Parallel Controller
2. Frontend performance testing. Rendering happens solely on the client side and therefore you don't need to have more than 1 thread running the real browser. Moreover the load test may be executed absolutely separately as they're not connected. Of course first you need to get the response from the server prior to rendering the page, but rendering time, script execution time, web vitals and so on happen on client side only. So you might want to consider a tool like Lighthouse or profiler tools which are shipped with your browser developer tools for identifying and fixing client-side bottlenecks and then use WebDriver Sampler for regression testing purposes.