We’re aware that videos embedded in posts aren’t playing for users on iOS and users of Safari on macOS, and that when they do play, they can sometimes take awhile to start. This is related to a forum software bug, and hopefully there’ll be a fix soon that we can apply.
(Details here if you’re interested: https://meta.discourse.org/t/issue-playing-videos-on-ios-and-macos-safari/387497/19)