Those passages are very small areas, so the 3D rendering engine knows that it has to calculate very few faces, vertices, planes... So when you enter the passage, the movement is suddenly smoother than usual. You may have been used to a lower frame rate outside, and therefore misinterpret it.
Or: You passed a "coordinate changing" portal. Here, the camera needs a little time to point back at your avatar after the world coordinate system changed. It is the worse the more distant the camera is behind your avatar.