Google Fiber, powerful MacBooks for the last decade-plus (currently an Apple Silicon machine). Normal gmail takes longer to do its AJAX requests than full-page loads on "basic HTML" gmail, consistently. Lots longer. It also likes to eat 400-500MB of memory and all the processor cycles it can get, sitting in the background.
Inbox was even worse, but I think they fattened up Gmail to match it after Inbox folded so the Inbox-loving people wouldn't suffer from increased performance when they had to switch back.
On the plus side they drove me to finally start using real, native mail clients again, so... I guess I can thank them for that.