This one is best reported and explained using a gamesave. At the view angle that the sim loads in, the FPS is great, around 260, but move around and after a certain angle, the FPS has a dramatic drop down to 15 FPS or so. When you go to the view area that it started at, it'll return to that high FPS. In fact, theres a point where it jumps from 15 or so to 260, causing a brief jump as you move the view around.
I've also added the output log.
Edit: Nvm, apparently I had the decals up at 1000, moved it down to 200 and the lag went away.
Although there is still a pretty big drop from 260 down to 70 or so.
Edit2: Even with decals at 200, I'm still getting periodic fps drops and lag while the sim is running. The timestep isn't going yellow or red, so it's not that.