Can you share your code?
Edit: I was coding it up now, but my storage is full (having trouble copying text to my clipboard and unable to save files) so I don't think I will be able to deliver on what I intended. Sorry about that.
On the HTML side, I had a code generator (in Javascript) that output `<html> <button /> <15000 more buttons.../> </html>` and wrote it to a file.
On the Flutter side, I used the default Flutter sample app (`flutter create .`) and I just used a similar code generator to put 15000 buttons in a Column widget.
I don't think I'll be able to do much else to help with reproducibility sadly.
If others try the same, I would be interested in if their results are any better than mine (with Chrome/Firefox being faster).
Of course, it's unfair to allow Flutter to have optimizations but not the raw HTML, but disallowing well-known, real-world optimizations is also an unfair test IMO.
But, that's a very web-specific UI design idiom that you just don't find in typical native apps. If you change your test to use desktop UI as it's intended to be used, and then stick say a few tens of millions of buttons in it, then the browser will find it much harder and the desktop toolkit won't break a sweat.