I have the same issue. If you teleport your ship to orbit, the FPS will tank above KSC and exactly in the opposite side of Kerbin.
If you see the task manager while playing, you will notice that the game starts to use a single thread at 100% (and GPU use drop).
The fix:
If you wish to flight without lagging, quicksave and quickload in launchpad before launch. Usually it works at first try. If not, repeat until you notice the framerate doesn't drop too much.
If you have mods installed, you can teleport your ship before launch (you will need clamps)
