There are some performance changes and big issues with main story creatures, which are detailed below. In answer to your questions:
1. Instead of game freezing for a few seconds when spawning lots of creatures, game lags and goes very slow until everything has spawned. The time from entering the map to when I can move normally seems to be slightly longer now.
Near the end of 2:9 at the dragon boss fight, the 5 dragons spawned in quite slowly one by one, instead of being all there instantly as normal.
3. No problems with creatures not showing up but several problems with talking to important quest creatures. I've come across these examples.
Cannibals of the Forest - Talking to Harmaatar doesn't trigger the final fight of the quest. All you can do it keep talking to him repeatedly.
3:9 - Can't talk to Rayan to trigger the final fight of the quest. Clicking on him does nothing.
Talking to creatures to start these quests worked as normal.
Edit: The quests above were new for my character. I tried replaying an old quest, 2:9, and all the talking parts worked out fine at the end.
1 issue though with creatures not showing up in this quest though. I accepted help from the Lord of The White Order but the 2 soldiers that you are given did not spawn. I did have a familiar and an Epic Gate creature at the time so not sure if that may have caused the soldiers to not spawn.