I hope it was the result of plenty of well-intentioned individual business decisions, such as Gmail now working offline in a first-class way, and relying on (presumably more complex and slower) web technologies to make that happen. I mean, the new Gmail is supporting a lot more features (including ones brought from Inbox), so a total architecture rewrite was probably necessary. BUT -- at the same time it's super-sad,…
I think that's partly what contributed to this slowdown --- faster JS execution to a browser is basically like faster hardware to a native app, thus reducing the impetus to optimise the JS code itself. Imagine if hardware wasn't getting any faster (we're close to that already) and browsers still used regular intepretive JS execution like IE6 instead of JIT'ing. If browsers don't execute JS any faster, then web app developers would have to do a lot more optimising to get even acceptable performance, and perhaps we'd see some more interesting tricks and knowledge develop.