AC+DC is always a single value compromised of sqrt(sqr(AC)+sqr(DC)), the question is how the meter does this.

Most meters reads a AC value, then a DC value and then do the calculation, i.e. reducing the update rate, a few meters may be able to do a combine AC+DC measurement in one conversion. When doing separate AC and DC measurement there is also a issue about the time needed to do the switch between them.