So…is that to say you've had issues with Mail.app in the past, and have tested it on El Cap and are still having issues?
Mail.app has been nothing but a complete disaster for me (my clients) since 10.9. (and 10.8 wasn't great). But in 10.9 and 10.10 it became nearly unusable. Stalls, no new mail notifications, PWODs, terrible performance, list goes on and on and on. IMAP, Google IMAP, iCloud, none of it mattered. It is not unusual to see threads in the Progress dialog crossing between events (which means the thing isn't thread safe!).
So I'm very interested in knowing if Apple has fixed Mail. Tim Cook promised Mail would be fixed in 10.10 at last year's WWDC, which meant they ACKNOWLEDGED the problem (for a change); it wasn't. The lack of acknowledgement of that in the 10.11 keynote was disconcerting. [It is so bad, users are talking of moving to Win 10 when it ships. They were switchers from XP, they WILL go back. Take note, Tim Cook!]