Very strange thing!

I'm using videoJS, a HTML5 video plugin for wordpress and I want to allow my users to upload a video file and then stream it.

When I'm on user blog and I put the tag with the URL [video mp4="http://blog3.mywpmu.com/files/2010/03/video.mp4"] it doesn't work but if I use the video tag with the URL [video mp4="http://mywpmu.com/wp-content/blogs.dir/3/files/2010/03/video.mp4"] it works!

I remember reading once that since the script is using a masked URL, the whole file is being "passed" through PHP which makes it slower and can cause problem, is this what is happening here?