You call your function every 50 ms. That is 20 times per second. That function uses “Move to Actor”.
From the documentation:
> Makes AI go toward specified Goal actor (destination will be continuously updated), aborts any active path following](Move to Actor | Unreal Engine Documentation)
So the destination of each AI is updated each frame based on it’s current path (relatively good performance because the path is only updated, not calculated anew) which you (unintentionally) abort every 50 ms and recalculate for every AI.
Pathfollowing is quite expensive and leaves lots of room for optimization in general. This alone could explain at least a large chunk of the performance loss you experience when spawning enemies.
So try changing that. Maybe that will take care of your issue already!