hi Christopher,
There are so many aspects of UI5 that are incorrect (in my opinion) that each day I read more I get something new to blame. My preference is usually something that provides me performance, meaning React, Polymer, Angular & Knockout - I've used with success and I always had decent results, being the worst part the ripple effect, but even thou, it would be around 30-40 frames... but again, there's are lot others I haven't worked (or used) yet.
About where it can be optimized, well - you open up any code you want and I'm sure you will be able to optimize - the problem is, a lot of touching in dom for no good reason (how the RenderManager works, other frameworks have virtual dom and can calculate diffs a lot faster and smarter), the CSS is over complicated for what it should be and really, the code is not "mobile first" as it's claimed because if it were the performance would be a lot better in general.
Other frameworks & libraries also implement virtual lists for "big lists" because it keeps the number of elements in the dom always small enough not to cause performance impact.. by now you should know it's basically impossible to bind a big item list in a mobile device using UI5, it crashes straight away because the dom is too big, and try to manipulate that.
I'm actually quite curious now, which other libraries have you used that are "inferior" to SAPUI5?
Cheers,
Dan.