You should start out by properly pinning down what exactly is causing your issues.
Check out this documentation page:
If your bottleneck is the amount of polys / animation. There’s no point in optimizing tick / gameplay further.
To answer your question regardless: Enemies don’t need to have an AI controller. You can implement behavior without. You can also code your own movement instead of using unreal engines nav mesh and movement code.
I find it unlikely though that this is causing your FPS to drop this significantly. Coding your own pathfinding / movement isn’t trivial and the chances that you’ll gain a serious improvement by doing that is unlikely. It certainly will have less features (because you would have to code all of that yourself) which will definitely end up worse in the long run.