It seems to be working fine when "Mail" is closed. Still lags but not as bad. I've tried clearing the data logs as well as reseting the NVRAM.I have a pretty base spec late '13 13" MBP, mine is running perfectly fine on El Cap. Maybe there's an app that's hogging resources? Check the Activity Monitor
I don't really have much in terms of apps, what do I looking for in the Activity Monitor?