I've been working on HTML5 video features on a website for several days now. I'm finding that an 800x450 (16:9) video at good quality plays smoothly (perfectly

I've been working on HTML5 video features on a website for several days now. I'm finding that an 800x450 (16:9) video at good quality plays smoothly (perfectly, in fact) when using a Flash player, but is choppy and slow in Safari using the HTML5 video tag. I've tried various bitrate, buffer, and quality settings, without any improvement. As a result, I'm now working on using Flash to play the video in all cases except when Flash does not exist, because this is just not working out for me. Maybe I'm just not doing it right, but it beats me what the issue is.