1. Reducing the shadow map from 2048x2048 to 1024x1024 was the first thing that caused FPS to increase to 30

2. Having the camera amidst the exhaust particles caused lag, possibly because of sorting, but I don't see why sorting would need to do more work

3. Removing bump and specular on 200 units

But now the shadows are missing on the trees. A minor detail, messed something up.

Whenever there is blending occurring multiple times over the entire screen, like when the camera is inside a bunch of particles, that typically causes a slowdown. Blending is expensive, which usually isn't a problem when it's only over a small portion of the screen. Especially since particles are drawn back-to-front, meaning there is lots of overdraw, which is unfortunately necessary for it to look right, but expensive!