Years ago I had this discussion with Peter Cattrell ( at that time Fay godwins printer). His and Fay's experience was that mirror lock helped at all speeds when a camera was used on a tripod. The discussion came up privately at a workshop when Peter commented that maybe one or two images I'd shown didn't quite have the critical sharpness he'd expect, they were made before I began always using the mirror lock except for hand held work.

In practice hand-held your body absobs the shock from the mirror but on a tripod it tends to be amplified.

Ian