"GMail is slow because Google can't afford to spend a lot on it. But people will pay for this. I'd have no problem paying $50 a month." Ok. Number of Paul Grahams in the world times $600/year = ? Most people on the web are ridiculously stingy. "I would pay for this" is a terrible way to think for an entrepreneur. Believing that what we think represents the masses is a rookie mistake.
I think email needs to be changed from top to bottom, but offering speed alone would be a hard sell to launch with. You can get the Microsoft offering (Exchange Online or packaged with Office 365) starting at $5 a user. Microsoft can afford to spend money on it. I've used it and it is a lot faster than GMail (although it wasn't enough for me to switch).
For the vast majority of the people on the planet, GMail is just good enough. Yahoo Mail and Hotmail are still doing well.
Perhaps email is perceived as a problem just because it gets tons of "face time" with us every day. I'd leave it alone and focus on one of the countless unsolved problems in the world.