According to this article on Lockergnome, Blogger service will eventually slow down on speed and performance because of its popularity in the web.
All I have to say is that so far, Blogger has not slowed down on my ancient PC and dial-up connection, so I'm not worried one bit. Another thing to note is that the Blogger staff is ready for this eventuality and are beefing up their hardware.
Now all Blogger needs to do is to support Opera 8 browser. Blogspot, please support Opera 8! I want Opera 8 to be my default browser! Firefox is slowing my machine too much! Blogspot, I'm gonna hold my breath until you support Opera!