Hi geoff, usually when you arrive at Guangzhou Airport by an international flight, you should collect baggage and go through immigration and customs firstly. This will take you about one hour at least during the peak time at midday. Then, no need to exit the arrival hall since you could turn to the transfer counter where you could get boarding card from China Southern Airlines and check in your luggage again. This will take you about 0.5 hours at most. Finally, you could walk to the domestic departure gate and go through security check.

For saving time, you could print out your boarding card online in advance if possible. It will save some time in case of a long queue at the transfer counter.

Hope this helps!