My Partner has travelled on China Southern and China Eastern a couple of times in the past two years to YVR and LAX and reported that in every one of her transfers at Shanghai and Guangzhou the line up at immigration was 2.5 hours.
Not PEK, but I was at PVG last week and spent 4 minutes in the immigration line - past trips have been an average of 10-15 minutes at most through there. Depends on the airport and time.