Ironically the vibration on a tripod is worse at speeds you'd think would be OK. By half a second it's becoming less important, it's the 1125. 1/60, 1/20, 1/15 which can be affected quite significantly.

The graph above is one instance, the frequency, mangnitude etc will vary depending on the camera, tripod etc. You need to test your own eqipment and see what works best, but as cameras age some may get worse.

I've seen very significant differences using the mirror lock on my Mamiya 645, and I know people with Bronicas, Hasselblads and Penta 67's with similar experiences, all when using good professional tripods.

Handheld is another thing, I've been happy shooting my M3 Leica at 1/30th great results more consistenly sharp than when I used SLR's, I prefer a TLR for handheld MF work when possible althopugh I do still use my Mamiya 645's as low as 1/30 with good results.