Most GA planes don't have an AOA indicator, so speed is the most indicative. That works for flying straight and level, anything else is a best guess. Most GA planes don't do aerobatics either.
So to get back on topic: in a straight and level slip with constant speed, do you think the speed at which the plane reaches critical AOA on a wing is higher, lower or the same? My guess is higher, see my reasoning above. Due to the air flow coming from a side angle, the indicated speed could be lower, higher or the same, depending on the position of the static port and pitot tube.