So far, Flutter is jankier than a webapp on iOS and on the web.

You could say the same when comparing native iOS apps vs web apps, and these days almost nobody cares. This jankiness will be solved --wether it is by optimizing flutter's core rendering engine or just because mobile devices will receive more powerful CPUs.

One way or another you will be able to build hugely successful multiplatform apps just using Flutter.

I hope so, it would be quite compelling if it was but so far I've been unimpressed and there's a lot of resources being pumped into the project.