The loaded video is actually not an MP4 but a MOV. MOV, 99% similar to MP4, is the container format of Apple’s Quicktime technology. This video only works on Safari, and in order to play this video in Safari on Windows, a user must have the Quicktime plugin installed. No other browser is able to play the video.
Flash performance is noticeably better in Safari on the Mac than it is in Chrome or Firefox — I did not know that. Video performance in Chrome for Mac — both HTML5 and Flash — is downright terrible. (YouTube ought to stop telling Mac-using Safari users to “Try YouTube in a fast, new web browser!” with a link to Chrome.)