I've being playing AC Brotherhood for the last couple of days at maximun graphics (all steady at about 100 fps or so). The thing is I noticed that when I enter any shop interface or press Esc to go to the menu, and then come back to game it lags a LOT. And I mean a lot: I'm talking of drops from 90-100fps down to 10 or 20. After a 10-15 seconds the lag goes away and i can play normally again.
I've also noticed that this "lag intermissions" are getting longer ever since they started.