I can open a 400 player FFA on
https://www.warzone.com/SinglePlayer?PreviewMap=100021 just fine on iOS 12.5.7 (most up to date version that iPhone 5s has, it's at least 8 years old).
The only real slowness I get is viewing full settings of games in browser like if there's lots overridden bonuses or lots of settings of mods being listed. I don't have that issue on iPhone 5s app, iPhone SE app or Windows standalone.