Mail.app does have settings that can help you delete mail that you've read from your remote mail server.
In Mail.app you can hit 'command' + 'comma' together "⌘," and this will bring up the Preferences. Then select the upper icon 'Accounts' from the Preferences pop-up, then select 'Advanced' (having chosen which account you're working on, if you have several). In the Advanced settings you can select various options such as checking 'remove copy from server' - after a certain time delay - one Month or One Day, or you can bonk the 'Remove Now' button which clears the server of read-mail more or less immediately!
Save your changes on exit, and it'll do the clearing automatically!
(In the past I have occasionally had to enter the webmail interface of some budget e-mail point of presence servers to clear them out manually, but that was years ago under OS X 10.2)
Getting back to iMacs, now that quite a few Ivy Bridge WinPC all-in-one's are hitting the market it would be great to see an Apple Ivy Bridge rollout in the 21.5" and 27" medium power desktops. Mail.app would then likely perform reasonably snappier.