I think it's a valid question..
I've seen Mail.app take 5 seconds or so to open a message with a double-click. No funny stuff either; plain text, short, with no attachments. It seems to get worse as time goes on, and nothing seems to make it better - empty trash/junkmail, restart the app, etc.. Why should it be getting slower?
I'm very tempted by GNUMail, but it lacks the bayesian filters. I get around 1000 spams a month, and having good filtering is essential. I may give it a go anyway, but it wouldn't be my main mail app..