This might sound strange, but it really does happen on my PC.
First I noticed this in no-win scenario, a couple of days ago. We've assigned each fleetmate to each direction of the map (NEWS) and a floater, and while I was looking how the north one was doing with the enemies (~50fps with grav wells and CSV, not to mention enemy fire), I turned my camera around to see how's south doing, and the framerate dropped severely just as the battle went offscreen. Mind you, only battle that was happening at the moment was on the north part of the map. Later on, I had to fight my incoming enemies with major graphical lag, since all the other battles were happening off-screen. My fleetmates reported to have the same issue.
Today I noticed the same thing in dyson sphere allied zone, I was just looking for a good spot to take a screenshot, when suddenly the framerate dropped. I turned the camera around, found someone fighting a bunch of Voth frigates, and immediately the framerate went back up to its usual 50-60!
Also, its inconsistent because it does not happen on every map!
It seems like a very serious coding/optimalization issue, though not excluding the possibility of issues with my own PC.
Specs: win 8 64bit, i5 4670, 8GB RAM, GTX 770.