You need to clean your inboxes if you want more speed with this firmware version.

I get around 700 messages per month (and no spam, so I need to archive them all) and usually keep all messages from one year in the inbox. I had around 7000 messages from 2009 and that was way too much for the N900. I cleaned it down to one month and that made a big difference.

Now it's getting slow again, since it's about a month since I cleaned