Fast e-mail client for OS X? I'm currently using Thunderbird for my e-mail needs on OS X. I manage 4 different e-mail accounts, one of which receives a lot of e-mail per day.

As the messages pile up, I notice that Thunderbird is getting more sluggish. Is there an alternative that handles large volumes of e-mail efficiently?
Nobody's mentioned Apple Mail. I love it, particularly the Lion version.