Scaling enemies like this, essentially having them act a level timer without the immersion breaking UI clock, is a really cool idea.
Great work getting a 3D game working on browsers.
I noticed that the last level can lag pretty bad once there's around 50 spiders. This isn't a lot of enemies, so I have to imagine it's the path finding algorithm slowing the game down (which I'm assuming is reasonably complex and being fired every frame). If I'm right, then you could fix this by reducing how often the algorithm is called from every frame to every second or three.