Automated performance testing

### Your use case

#### What would you like to do?

Run performance tests against key functionality of the application (e.g. rendering, room switching) as part of CI.

#### Why would you like to do it?

Element has frequent performance regressions. We should guard against this to prevent it occurring; its often only detected a while later and its really difficult to tell which change caused the regression

#### How would you like to achieve it?

We could do this in Cypress by simulating scenarios that cause perf issues e.g. many rooms, many events in rooms etc.

### Have you considered any alternatives?

No

### Additional context

_No response_