So it looks like GPU is the bottleneck in your case. Dynamic 2d lighting can be computationaly expensive and as every enemy has it's own light source, that's why your framerate drops with more enemies on screen. Also, it looks like 2d lighting in Unity is still an experimental feature, so it's not really well optimized. And in terms of gameplay, you have limited time before possesed enemy slowly starts to resist and move on his own - that's probably why the movement felt random to you.
I know that the enemy resist, i struggled in a level and at the start i kept on doing the same inputs, it was the first 3 seconds so either the movement is really sensitive or resisting happens all the time just a little, and also the enemies felt random too. I know my GPU is a bit old but still i dont think it should lag that bad