I made a head model with a total of 7.5 million faces, wrinkles and all. In object mode, the viewport is horribly slow. In sculpt mode however, it runs very smoothly and I’m able to pan and rotate around the viewport very easily. These are at the same multires levels by the way. Why is the viewport so much faster in sculpt mode?
Not much of a sculptor, but if Outline Selected is enabled (the default) in Object mode it will slow things down. In the N panel, under the Display tab uncheck Outline Selected, or enable Render Only maybe, and see if that makes a difference.
That’s odd, is it possible you have a subsurf after the multires in the stack? If that’s the case when you sculpt it ignores the lower down subsurf, but back in object mode it would use it. At those vert counts a single subsurf could bring down the ship.
The drawing code that powers sculptmode is far more optimized for large polygon counts and makes use of modern drawing routines (while object mode uses archaic fixed-function routines from OpenGL version 1.1).
Optimizations for object mode is coming though (there’s an active branch containing faster object and subsurf drawing).
Now there is a legitimate answer!