I still use the basic HTML version, it works fine and does all the things I need a mail client to do (except a "select all" button, which I've added with a short userscript: http://ix.io/3pXu/js)
It’s embarrassing quite how much faster the plain html version is. Proof that all the fancy JavaScript gubbins do very little to enhance the experience and a whole lot to slow it down
It is now, but it didn't used to be. Gmail at launch was incredibly fast. It gradually got a little bit slower over time, and then they made it a lot slower with a rewrite a few years ago.