Generally, Flutter applications are performant by default,
so you only need to avoid common pitfalls to get excellent
performance. These best practice recommendations will help you
write the most performant Flutter app possible.

Note:
  If you are writing web apps in Flutter, you might be interested
  in a series of articles, written by the Flutter Material team,
  after they modified the Flutter Gallery app to make it more
  performant on the web:

Optimizing performance in Flutter web apps with tree
shaking and deferred loading

Improving perceived performance with image placeholders,
precaching, and disabled navigation transitions

Building performant Flutter widgets