Gmail has nearly 50mb of JavaScript that thrashes I/O so hard that clicking the search bar and typing too soon will cause it to skip letters (because the search bar is probably some Angular-powered abomination).
It is and was a regression from the previous design which was fast even on 3G connections.
It's driven by Google engineers running their bloatware on i9 MBPs with 64gb of RAM. Oh Gmail is slow? Have you tried not being poor?