I dunno. They cutaway from McCoy before he starts to answer Mirror Spock and the next time we see him he needs someone to hold him steady. Scotty guides him up the transporter steps and holds onto him all the way up till Spock beams them out.
McCoy doesn't seem to snap out of it till after he's back in the regular universe.
Valeris seems to have snapped out of it relatively quickly when she answers Kirk that there is only the prototype BOP.