For frontend performance: record the frame rate (via requestAnimationFrame, look at simple frame rate counters), memory, count of DOM nodes on the page during each of your integration tests as a good start. You can get a lot of this via window.performance and memory info you need to pass an extra flag into chrome headless.

It’s also useful to instrument this in your production environment (called real user metrics) for some % of user sessions and combine it with more synthetic tests.

The backend is of course really easy, just measure request times (min/avg/max) and time to first byte.

Edit: Don’t know why I’m being downvoted for this answer? Lol