What do you mean slow? When it updates? It is a bit of a hog when it updates and slow - depends on CPU and internet connection I guess, but you can cancel the update and update manually when it suits you.
I think everyone is talking about
updating the Genius database, here. It seems to be fairly slow irrespective of speed of connection, having primarily to do with library size. And whether it's
objectively slow, I think it's at least reasonable to say that it's among the most time consuming things iTunes does -- it takes longer than anything else I do with iTunes -- importing music, getting artwork, syncing iPods or iPhones, etc.
For a test, I enabled it with a very small library of about 20 songs, on my work computer, and it was quite rapid. In contrast, when I add one or two songs to my home library of around 4000 songs, it takes a
long time to gather results and send them back.
Also, one way or another, one
must update whenever one gets new songs, because otherwise Genius will not put them into genius playlists. It gets even more annoying with the iPhone -- if I buy new music on the iPhone, in order to get it into Genius, I have to sync the iPhone to get the song in iTunes, then update the Genius database, and then sync again to get the Genius info for the song onto the iPhone. This really extends the amount of time syncing my iPhone takes.
And I
love Genius playlist, and I use it extensively, which makes this all the harder....