Performance
- Single-threaded CPU performance
- Multi-threaded CPU performance
- WebGL performance
- Video performance
- Network performance (how long does it take to transfer data to various locations, what's the lag, is the lag consistent, etc.)
- (Maybe) Time it takes to execute certain JavaScript functions
User behavior
- How does the user use their mouse when navigating web pages?
- Not at all?
- Jerky movements?
- Smooth movements?
- If the user uses the keyboard, do they appear to be advanced keyboard users, do they have an IME, etc.
- Does the user press X buttons on tiny annoying popups that wouldn't interfere with the page's browsing experience?
- Does the user appear to block access to certain resources? (ad blocker)
- Does the user's workplace/country/etc. appear to block anything?
This is why I stay attached to making simple HTTP apps that don't require JS, but this is clearly not the direction the web is going at this time.
[1] https://twitter.com/davywtf/status/1132026581038190592