What causes a mobile app UI to run slow or laggy? and Can a framework cause an app UI to run slow or Laggy?

Too much shit in event handlers probably.

Framework overhead may be. Keep UI thread free from long running tasks.

What OS and what language. If you are using a scripting language you are very far removed from the machine and the layers of support code costs execution cycles. Python is roughly 40 times more processor intense (which if you hit your resource limits means "slower") than C for example.

Have a look at https://web.dev