Thank you for your feedback! It's true that I put a lot of my energy in the environment and I'm glad it shows.
Concerning the lag I wasn't able to pinpoint where it come from in such a short amount of time but I think it's related to the first time a sound being played and maybe a lack of optimization of the light count.
The spiders movement is a bit simple, it calculate the distance between them and the player, at a large amount of distance is goes back to it's starting point by using a navmesh, at a middle it jumps (certain spiders jumps certain just don't) and at a short range it follows you in a straight line.