The gnoll encounter is the first in a pair of encounters with a very similar set-up: "One leader-dude and a pack of identical mooks". In this case, a gnoll Demonic Scourge (Brute) leading gnoll Claw Fighters (Skirmishers). The other case is the ornery bugbears I first hinted at back in the cell block. If you want to cut down on encounters, one of these is a good candidate.
As it is, this encounter works out okay. The Claw Fighters get a relatively big room to zip around in, which they thrive on, and the Demonic Scourge hits like a mofo (it's not very sticky, but rather relies on being a legitimate threat to tie up PC:s).
The aftermath of the encounter is interesting too. The last standing gnoll will offer information for freedom. The module even suggests that future encounters with gnolls in the area should be easier to reflect how you showed them the weakness of the Temple of Elemental Evil. There are, of course, no further gnoll encounters listed in the module, but it nicely sets up future adventures. Maybe the gnolls could even be grudging allies?
Either way, this is the place to tell the players where everyone is, if you haven't already. If they didn't use shenanigans to bypass the walls, they have at most three encounters left, and that's assuming they came from the north (through the ornery bugbears, I'll get to those alright), leaving the giant crayfish, the ghouls and Lareth's office still standing.
Oh right, Lareth. Just a little peek into the bugbear den and we'll get to him.