It would be pointless to leave feedback for someone because they payed instantly. It's more so that you can let other sellers know that this person is a good and easy buyer, or, in some cases, if their demands are too difficult to meet or something. There wouldn't be a negative option for this otherwise.
Also, as for buying items from china, it was recently chinese new year which has affected their postal service. I bought something about a month and a half ago from China. The seller usually got the items to me in a week from shipping, but this time it took over a month because of the new year. Some sellers, however, do take longer, but buying from China, you kind of have to expect it, from experience. It's worse on Ebay, I've found, but from everyone, probably because not all ebay sellers take the care over their shop that etsy sellers do.
It's best to just wait and leave feedback when you receive the item and the transaction has been completed. The feedback option is put up immediately after purchase because Etsy won't know when the item arrives, and some sellers sell digital items and are able to deliver like 15 minutes after the order has been placed.