I have an external non-profit wiki hosted under Radlines.org. The problem is, it takes 40+ seconds to save changes made to articles with many images from Wikimedia Commons such as this article:
It does not occur when saving changes to an identical article with only locally uploaded images such as this example:
I also created an identical test page at the free wiki farm Miraheze, and it only took 5 seconds to save changes to the page there although all images were located in Commons:
So both Miraheze and my own connection thus work fine. The problem seems to be in the communication between Wikimedia Commons and my host (Civihosting.com). They tell me that there's no problem with their servers, so can you do anything to decrease this latency?