Yeah, it's definitely fast, which is really neat.
Obviously, there are quite a few graphical and functional omissions with which I'm sure you're familiar, and that you warned us would exist. As far as actual performance goes, here are some things that I've experienced:
A lot of the time when I try to bring up the map, it'll give me a blue screen. Clicking around still takes me to a room, but I can't really tell where I'm going. This is fixed by reloading the client.
Attempting to use the scroll wheel in the guest room or friends menus causes them to go blank. Clicking on the scroll bar on the right side brings them back. The scroll bar is also EXTREMELY sensitive.
Character sprites will often have their hair / hats disappear and reappear. Sometimes they also appear to not have eyes. These are particularly noticeable when dancing.
Sometimes, game alerts will pop up transparent, as opposed to their typical blue. This usually coincides with other performance glitches, and is also fixed by reloading the client.
There seem to be more invisible cannonballs in the pirates minigame. It seems like maybe the cannonballs that snuck through the trees on the old client are not visible on the new client, but still do damage?
Sometimes, when playing pirates, the health/ammo pickups and any shot cannonballs will be completely invisible. Also fixed by reloading the client.
Also on the subject of pirates, you have to click in the chat box in order to chat during the game. I very much preferred just being able to type.
It seems like it's not possible to enter the default instance of a room from advanced mode. I do, however, like that it's possible to see exactly how many people are in a given room.
It seems that control is no longer the button used to activate ghost vacuums in the HM minigame, and that it switched to shift, which activates the sticky keys. I also glitched out of the only game of HM I've tried on the new client, so I don't have a ton of other feedback on it, except to say that the new ghost sprites look really nice.
Overall, I'm reloading the client a lot to combat various glitches. But it reloads really quickly!