How can I test the time it takes to load and render my application?

I experience once in a while that the applications I create get slow over time. It might be due to for example growing amounts of data, more users and deployment issues.

I would like to setup automated tests of the time it takes to initially load the page.

I sometimes use Locust for load testing. But my understanding is that it only measures the response time of the initial request. Not the web socket communication and rendering part.

I would like to to test the time it takes before the page is rendered and ready for the end user.

Furthermore I would like to setup some max limits and get notified when the time exceeds the limits.

I’m imagining I could test this using Pytest and run the tests automatically on a schedule.

But which tools would enable me to do measure the time it takes my application to be ready for use?