I don't know about this... I use tumblr and it usually seems pretty slow to me... normally more than the 3sec load time they quote... haven't tried posterous yet....
Good point. Tumblr is a lot faster than Posterous. However, it is only at about 50 percentile comparing to other sites over the Internet while Posterous is below average - in other words, Tumblr's performance is only about average. Both have lots to be improved.