I have N900 pre production device from summit and media player with 17G of music (about 3400 sogs and all with album art). It took some time when I first time started media player but after that I haven't seen any big problems. When I start media player and browse all songs processor tops for about 10 seconds and after that I can scroll through the list without hiccups. Browsing albums there is also similar start up delay but after that it also works great. For albums or genres delay is bit shorter.

Generally speaking one can live with it but I would like to see little speed bump in media player performance when there are thousands of songs. And remember that this is pre production device so performance might be different on final release.