Hi Michael,
So I just want to be sure you understand. Using your last scenario, isnt the issue.

I perform your last scenario of drawing a sphere and starting the line command. I "Do" see a performance difference with snaps on and off, but it is very slight, and on my part, expected and "works fine".

Here's the difference:

Turning on a command, actually "fixes" the issue I was refering to. With just a sphere, and "no command running" (Selection mode), the erratic behavior is present. The rotation and panning "jumps" with large delays in movement (When doing simultaneous movement) In a single maximized viewport only (all of them, whether its front,back or 3d etc)

So to recap, Only in selection mode with a 3d object, not performing anything other than navigation with simultaneous device movement.