This is not a Flarum bug. This is related to connectivity. As not all posts are loaded at once (that would take ages), xhr/ajax calls are made to retrieve posts that scroll into view. This xhr call most often works as fast as possible on your connection, but sometimes they are delayed because of DNS resolving, ISP request throttling on your end or alternatively DNS relaying on our end. I can imagine that CloudFlare - something we use - might influence the requests you made.
