My FPS went down from 180+ to about 120. That's a big difference for just a few new buildings... bug?
Considering your eyes see at an average of 20-24 FPS in real life......when "applied" to the format of FPS. Or to be more technical, 50 cycles per degree is the upper limit of an exceptionally good, healthy human eye. (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m)
As someone else said, worry about it if it goes to 30 FPS. Your eye just can't see any difference past between 60 and 80 FPS (depending on many factors), and your brain is honestly more than likely only interpreting 5 frames out of every 10 at 180 FPS. There's just more information than the visual cortex can process and deliver, so it "shortens the film" at such a high frame rate, and starts throwing out frames.
Unless your frame rate is a bragging point for you, I wouldn't worry.
Also, as slapshot said, make sure your Vsynch is on. If it's off, you'll get a lot more rubber bullets, and have an amazingly high FPS rate at the same time.