I had exactly the same problem, and all I could put it down to in the end was fill-rate, and a mouse bug which may or may not have been fixed recently.
I wasn't using a terrain at all, but I had exactly this problem of big slowdown but only when I was close, and ONLY at specific, quite narrow, angles.
If you're using mouse control at all, you might want to bung a loop of small delays ( I think it was suggested I use a loop of ten delay 2 's but I'm not 100% ) as the mouse buffer seems to get really hinky when the fillrate starts to become a problem.
Other than that, you can't really block certain camera angles ( well I couldn't anyway ) but you might be able to stop the camera from getting too close to objects.
I wasn't using a terrain at all, but I had exactly this problem of big slowdown but only when I was close, and ONLY at specific, quite narrow, angles.
If you're using mouse control at all, you might want to bung a loop of small delays ( I think it was suggested I use a loop of ten delay 2 's but I'm not 100% ) as the mouse buffer seems to get really hinky when the fillrate starts to become a problem.
Other than that, you can't really block certain camera angles ( well I couldn't anyway ) but you might be able to stop the camera from getting too close to objects.