I guess you don't browse any modern webpages? Otherwise I can't explain your perspective. You basically lose up to 95% of performance in anything remotely resembling a JS heavy web app. So it would run the same as on a cheap Android phone.
My experience has been the iPhone chip is fast enough to still run pages just fine. Maybe those ones that load 300 adverts and relay out the page 5 times as they load won’t work but they hardly worked before anyway.