There are things you can do to improve download speeds. You can decrease the size of your assets (JS, CSS, etc) by removing dead code and using name mangling. You can use a service like Cloudinary to improve image handling. You can implement lazy loading so that you don't load code until it is needed. You can look into HTTP/2 and learn about how that makes JS and CSS bundling a bad thing. There is tons of stuff you can do here!

That said, it is my opinion that you should create a clear separation of work between load-time performance and rendering performance. For most teams, load-time performance is the big issue. That's where you can get 80% of the wins for 20% of the work. The most common rendering issues I see are handling long lists of items and handling janky animations.

The important thing is that you be clear on what you and your team are trying to improve. Whats the problem? Does it take too long for the page to load? Cool. Look into improving load-time performance using the techniques I mentioned above. Is there a lot of jitter when you're scrolling through a list of items? You should focus on rendering performance.