The black background in the recent app list may be a JB thing. I have it on my Nexus 7 as well.
For those that are experiencing slow transitions on JB, i noticed that the update resets the speed preferences for window and transition animations as well as the animator speed back to the default 1.0x setting under developer options (in settings menu). Well it did on my phone anyway. I changed it back to 0.5x for all three settings and the transitions and window opening animations are back to being lightening quick.
I still get the occasional lag from the stock launcher, but that probably more of a launcher issue than a JB one.