Optimization advice horde wave shooter.

I am making a horde based wave shooter and I have noticing major FPS dips when a wave goes on for over 30 seconds.

The enemy being spawned is under 900 vertices and has a tick rate of 0.15 and only has two animations attached with a behavior tree that has three nodes.

Nav mesh rendering is static.

any advice would be helpful because I know it’s doable because of games like L4D2 and space marine.