Actually I believe huffYUV doesn't really have too much data, I think the problem could be that it's more complex then other codecs. Well I guess you could say that there is a massive amount of data.
Usually you'll get a file size reduction of around %33 or so when using HuffYUV. There is still alot of data to be processed and it isn't a simple task of copying it onto the screen. It has to be decoded and then send to the screen. So you have alot of data moving off the hard drive.Which when you access the hard drive that takes some cpu time, so the more hard drive access your use requires the more time it will take. Then you have to decompress all that data, which takes alot of cpu time, and then it has to goto the screen.
So with all those factors it can get slow :\
Your best bet is to use huffYUV as source, and then some lossy codec, like MJPEG or DivX even(provided it doesn't crash

) for a preview.
~klinky