Runs really fine on a low-end phone from 2019 with Firefox. Mac issue?
Runs fine on an iPhone.
I wouldn't say "very" for me but it's definitely not as smooth as your average webpage. This is pretty edge casey use of HTML / CSS though, it's not surprising that browsers wouldn't optimize for it since this is more of a stunt than the best way to achieve this output.
Runs fine for me on my mac devices, even on an old iPhone.