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).