I dunno if I have the exact same problem, but I've found that enemies who can't do anything active (i.e- attack) take a long time to finish their turns. This happens a lot if you slap down a big control spell like Hungry Hungry Hadar (Hunger of Hadar) or Sleet Storm; the enemies try to avoid it but it's like they get stuck trying to figure out how, or they end up dashing but not actually going anywhere.
I even saw an enemy action surge so it could triple dash, then just stayed still for 5-10 seconds and ended its turn because there was some slippy ice in its way that it might have made it over. It massively slows combat down, and actually makes the spells even more powerful because when most/all of the enemies bunch up around the edges rather than trying their luck at crossing it makes them easier to safely hit with Ice Storm and other instant area effects.
However, spells like Spike Growth which have a smaller area just cause enemies to jump past the area; I wonder if it's that jumping behaviour that might be the problem? Like they're trying to find a way to jump past it but can't? It's a super annoying behaviour anyway because it makes Spike Growth worthless against 99% of enemies, and overpowers other control spells while slowing down combat a lot.
The "AI" behaviour in dealing with area effects in general is a bit wonky; I also somewhat recently did the steel watch foundry in Act 3:
In there I found saving the gondians there basically impossible, because when the steel watch constructs go into their self destruct mode they don't seem to want to clear out of the way, and that's if they aren't getting themselves killed by suicidally teleporting towards groups of enemies that then curb stomp them to death.
I gave up on saving them entirely as I just didn't have the resources to do it properly (I'd have needed a fully hasted party and a caster who can slow most of the enemies to even stand a chance at rescuing them all, and even then I'd probably need someone with flying moves and dimension door to be able to save them from themselves.
The only one I could save was the one I had control over. I really wish Larian would give us control over all friendly NPCs (at least the ones that are fighting), as keeping Jaheira alive in Act 2 during the attack on Moonrise towers was similarly difficult, I ended up having to cast Otiluke's Resilient Sphere on her because she seemed determined to rush into the densest enemy group and die as fast as elvenly possible.
Ah the joys of "AI" behaviours.
