IE7 is out (not yet in Japan). Firefox 2 is out in few days (world wide). Lorelle’s article is a must read for the WordPress users.
Basically, both the wordpress.com and the wordpress.org uses lots of JavaScript and Ajax. Especially now, as there are lot of plugins and quite few themes out there utilises these techniques. These can greatly affect the performance of the browsers, and the performance worsens if it has not been well coded. In particular, the memory usage bit.