Thanks for playing!
I get the lag too, also after about 50 spiders. I only noticed it after the deadline. The spiders just travel in random straight lines regardless of obstacles, so there is no pathfinding to speak of. Unfortunately it is Godot collision handling which is the issue - 100 big spiders is a lot worse for lag than 100 small ones. To fix this I would need to look at a different physics engine, I think.
Thanks for the feedback and I'm glad you liked other aspects of the game. I note you didn't mention the awful tank controls! (I honestly would improve the controls if I continued to work on the game or did another like it)