I would consider Flutter apps that have been compiled to ARM and runs on an iPhone natively (basically just using a canvas to draw pixels on) is very fast. However Flutter web is not very fast, but it's getting there.
Judging from the comment by Hixie on Feb 20 it seems like it is not trivial to fix. For now I would just avoid animations. If you're building end user applications that have high aesthetic requirements then you should be coding using iOS and Swift. I say that as a Flutter developer.