I find mail.app bogs down getting "caught up" downloading my email if I don't keep it running. You can keep mail.app running without a window by launching it and then closing the active window. Mail.app stays underlined in the dock and the next time you need to open it, it comes up quickly.
I also use Thunderbird from time to time. I also use the webmail interface for gmail in the browser (chrome).
I also use Thunderbird from time to time. I also use the webmail interface for gmail in the browser (chrome).