I stopped using it for a while when its performance with GMail accounts went bananas. It kept thinking there were millions of messages to download.
When I installed 10.11, I figured I would try it again. For the most part, it is improved. It for sure looks like that problem was fixed.
That said, I still hear the fan on my laptop a lot and look to see Mail chewing on the CPU for no good reason.
I have mails going back to 2004. I wish this app was smart enough to not download everything. Maybe that's not the reason it's performance is abysmal. Before ditching it again, I thought I would ask for some opinions. I could move all the mail to a box and archive it, but then I would have to go delete it from Google which seems insane. This does not seem like a problem that should have a company of Apple's size tied up in knots over the span of many releases.
Maybe they let the Adobe guy who did Flash takeover mail when he joined and burning up millions of dollars of energy doing nothing is just habitual for him.