Ok, I saw it and this also happened to another player as well; you found a real problem. When enemies spawned faster than your build could kill them, the crowd kept growing until the browser couldn't keep up. I just pushed an update that caps how many enemies can be on the field at once, which should stop that from happening. If you still hit slowdown, tell me roughly when and I'll dig in.
And congrats on the win!
Also in today's update(using this reply as a pseudo-changelog, sorry :D):
- Elites arrive later, at 8:00 instead of 6:00. They used to show up at the same moment the game got hardest.
- The hound ambush around 4:30 on the docks map is smaller. It was the single biggest killer in the first five minutes.
- The death screen now tells you what killed you, with a one-line tip on how to deal with it.
- Browsers running without a graphics card switch to a lighter mode on their own, with no fog and a lower render resolution, so slower machines can actually play.