I also experienced a major stuttering on Uthelaim castle. It seems to be related to troop pathfinding or their AI, since all enemy infantry was stuck in formation behind the gate (I've breached both walls and didn't destroy the gate) and when I've killed enough of them so that they finally broke the formation and started runnning, the game suddenly started working fine.
Game version: beta e1.5.2
Win 10 64bit
Geforce GTX 1660
AMD Ryzen 5 3600
16GB RAM
SSD drive