I'm actually using mu4e for exactly this reason: It's so much faster than any web client could ever be. And I'm saying this as a professional web dev^^ And yeah, I know GMail - I was an early adopter and have seen two companies migrate to it in the last five years.
Of course, running mail within Emacs has its additional awesome benefits, but that's a different kind of argument I'll leave out for now. I'm honestly curious why people think/believe/know that GMail is faster than a well engineered locally indexed app. It just doesn't seem to be the case for me, but I hear this time and time again.