It could be an artifact of the codec you selected to render the AVI to before uploading. Try rendering to a file and using the same AVI settings to compare. It could also be that your computer plays the preview slower because it lacks the CPU power to play in realtime, but most modern computers will never run into issues like that, so it is unlikely.
The different codecs do make a difference in quality and/or speed.
Downloaded DivX as recommended in Morpheus tutorial and after playing around with the quality/speed configuration finally got it to run at about the same speed.
Just an FYI - after playing around with this a bit I found that reducing the timeline from 30 fps to 24 made it run exactly the same speed on YouTube as it does on my computer.