How do I change the mouse sensitivity for orbiting? When I rotate the viewport with Mouse3, the view rotates about twice as fast as I'm used to, and I would like to reduce this. I tried the "mouse sensitivity" setting and "NDOF orbit sensitivity" but neither had any effect.

It may be because my Windows display DPI setting is causing the pixel delta on my mouse to be doubled. Whatever the case, I'm really surprised there is not a slider for this, at least as far as I can find.
This has finally been resolved in the latest builds of 2.81 (tested with 2.81-0812949bbc3d-windows64). There is now an option in Preferences > Navigation > Orbit&amp;Pan > Orbit Sensitivity to adjust viewport orbit speed. It appears that the default speed has been reduced as well (checked against 2.8)