The thing is, Session plays essentially just gives you control of an NPC. The problem with this is that over the years mobs have been buffed and their stats increased to make up for the fact that classes have generally become stronger over time.
But the issue doesn't just stop there. Some NPC skills are completely bugged or just haven't been scaled. You can see traces of this in monster play.
Also just to make matters worse, NPC skill animations are as slow as they were back in SoA, in general, the original session plays haven't really been touched on at all since when they were released. It's archaic and old and I don't think there'll be a lot of focus on fixing them, as long as it's technically possible to complete the quests it's just one of those things you need to rough out.