I used to have the same experience when changing to Voicing 10; however, the only preset currently using V10 no longer seems to have this setup delay. Inexplicable. Magic!
I do not have the answer, but, per intuition, believe it is either the Vactrols [that controls the tube's operating conditions] starting to take a looong time to go from one setting to another, or some capacitor(s) related to those setup circuitries, that are down in value.
And the debunker: To my knowledge, Vactrols aren't typically reported as going half-dead with age, so.. we're back to the guessing game - or better, elementary electronics measurements..
And I can't use my mini lab, as all is defunct due to selling my apartment.
EDIT: While Vactrols may not go SOA (slow of age), they're often referred to have slow reaction times, and some reports indicate this is often a matter of the surrounding circuitry not properly driving the Vactrol, so, could be I'm not off on thin ice after all..
Potentially interesting read: https://groupdiy.com/index.php?topic=43011.0
Methinks it's the surrounding electronics causing problems for [some of] the Vactrols.