Why is Safari HTML5 video so bad?

i vaguely remember a time when HTML5 video loaded way faster than their Flash counterparts, but these days, whenever I queue up a video, I end up having to copy the URL and paste it into Chrome so that it'll load in Flash.

What happened to Safari in the recent builds that have made it terrible at processing HTML5 video? I just don't understand why it went backwards so much. For reference, I'm running the 10.10.2 developer beta.