Karl_Houseknecht's post in Hd500 And 500x Patch Change Lag was marked as the answer
"Heard about" and "experienced" are two different things. Even though there isn't spillover between patches, I haven't had a single problem changing patches within a song and noticing any lag. I'm usually very picky about this. Doesn't bother me with the 500x the way it did with the Axe-Fx.