It looks to me as if the HTML5 engine on Apple devices tries to preload either all, or a very large fraction of, a video file before letting you start play, much moreso than other operating systems. With a relatively small video file (50 megabytes), on WiFi, I often have to wait 10 seconds before a Play action works. I don’t see this with Windows browsers or Android browsers. I do not know if this is related. Seen with both seamless and non-seamless publishing. I’m actually thinking I need to detect iDevices and put up a “wait for loading” animated GIF, something I haven’t done in 10 years for any application.