My processor runs at a constant 3.95ghz, boosting is disabled, so the performance impact is not as a result of frequency change behavior.
The impact occurs regardless of in game graphics settings, shadows off, Vsync/frame limiters off, and appears to be associated with areas having verticality or heavy sloping.
Very little needs be present on screen to produce the frame rate drop, and sometimes, rotating the camera in a single axis only a couple of degrees can completely eliminate the input lag and return performance to normal.
Reference photo for location, along hillside at Brink (this is not the only location that produces this issue)
Set 1:
Image of rocky peak, local performance minimum, stable 57fps, significant input lag
Image of rocky peak, counter-clockwise several degrees, stable 80fps, minimal input lag
Image of rocky peak, counter-clockwise several additional degrees, stable 89fps, minimal input lag
Set 2:
Rocky peak horizontally aligned, stable 81fps
Rocky peak horizontally aligned, stable 93fps, camera shifted southward a meter or two,
Set 3:
Southmost position, 72fps, minimal input lag
Shifted northward, 1, 55fps, significant input lag
Shifted northward, 2, 60 fps, noticeable input lag
Shifted northward, 3, 72 fps, minimal input lag
I don't really know what could be causing this issue - the fact that it seems to localize in regions with a lot of terrain height variance is a good clue, though. Speaking from near-total ignorance, perhaps there's a flaw or an extraordinary inefficiency in how the game computes terrain slopes and peaks, either on the CPU, in the generation algorithm, or on the GPU and how those vertices come together?
These photos also only express the impact when the camera is highly zoomed in - pulling the camera back and showing a wide field containing a lot of these peaks can reduce framerates into the 20s, with enormous input lag - the game is only playable by constantly refocusing the camera to avoid looking at these regions.