Earlier quoted context omitted.
I've tested on my phone (1yr old pretty good Android one) and the whole thing is very laggy runs with like 10fps. What about a11y? The whole thing feels like Adobe Flex in 2010. It solves a great deal of problems that the web had but introduced a plethora of others.
[Flutter Eng. Dir. here] Still relatively early days for Flutter Web, so I would not be shocked if it's not buttery everywhere. However, we would certainly love to learn more. fluter.dev/support has links as to how to file an issue if you're interested. https://flutterplasma.dev/ is one demo to try. We expect to be updating flutter.dev/web and flutter.dev/showcase to update more over time.
Not up to the speed of well-written Javascript, but definitely viable.
In contrast, this is completely unusable on other browsers or on lower powered devices.
On Firefox desktop, initial loading brings a "This page seems to be stuck" prompt. Once it finally loads, clicking the play button just does nothing.
Please note that for 80+% of websites/apps, Firefox is faster or on par with Chrome for me. Google products and some other exceptions are the only ones that are consistently slow on Firefox. Whether that's intentional or just a lack of testing, I can't say.
Similarly unusable on Safari.
On lower powered devices this just freezes Chrome for me.
As I mentioned in other comments, I like Flutter, but Flutter Web is not anywhere close to being viable for production use.