We currently put all Adyen IPN messages onto the same queue as they come in, then process them one at a time. This process takes a few seconds for each one since we make a couple of API calls. Unfortunately, that's not fast enough to keep up with our generous donors when we set Adyen up to receive half of the traffic from US banners. This morning's test saw a constantly-growing Adyen queue, which only started dropping when we turned banners off. Donor TY mails were up to 1 hr late after three hours of the test.
I'm concerned that with scaling up transaction volume (if we go w/Adyen for EN6C cards for example) we might see an increase in donors asking if we received their successful donations. If we see that in Q2 I might ask to prioritize this task higher.