If you bought some cheap stuff from China stores on Aliexpress or eBay, the seller can provide China Post shipping and HK Post shipping. So the question is :which one is faster? This depend on the destination country and your luck.
Sometimes if your goods have been detained by customs for inspection, this may cause a delay. If you are very very lucky, everything went smoothly, sometimes you can even receive a China post package in 7 days.
The shipping rate is low
Actually, both this two shipping services take more than two weeks to deliver a package. The shipping cost is not expansive, that's why some eBay or Aliexpress sellers provide free shipping with China Post or Hong Kong Post.
Estimated delivery time
China Post Air Mail, 15-45 days
Hongkong Post Air Mail, 15-39 days
Both of them are slow, but if you have to choose one, better choose Hong Kong post. Sometimes China post took 2 or even 3 months to deliver a package. Hong kong post is more stable and has less problems.