NERD TALK ALERT!!:::
The enemies are actually pretty solid, what ends up lagging the system is the path finding and the enemies trying to not going into each other(the other agents avoidance) which runs on the main thread. as for the enemies they are actually animated baked, so it's full on the GPU and it did have an issue with the custom speed which created separate draw calls for each but for around 100-300 enemies that's fine(And I've fixed it already), so the issue i will be working now is create a separated threaded or async path finding of my own for this kinds of games so it doesn't bottleneck the main thread. for the web build i even took away the shadows and lowered some other options but calculations for pathfinding and avoidance between agents are a pain when you enter this world of like 400+ enemies all trying to reach you on the web build as a unity project.
All this to say: I'm working on it and will improve! Thank you for the feedback!