Since I GV a lot I've noticed this since the start of AH3. I just presumed it was an artifact of the dynamic tree detail algorithm. It's noticed from GVs because they are close to lots of trees. Trees at a distance are rendered at a lower LOD (level of detail) than those close. If you zoom the view then obviously those distant trees must be rendered at a higher LOD, and, of course, the gunsight view in a tank is zoomed. While the FOV is now less, the depth is more, i.e. there are a lot of trees at the relevant distance to render in high detail, so the frame rate drops.
If you really want to see this in action go to a port (on a terrain with, I believe, the Mediterranean texture set) and go to F5 mode and look at the trees there from various angles, distances, and zoom factors. On my system my frame rate is pegged at 60 in the air but can go down to the middle teens looking at the port trees in high zoom.