qt won't play .mpg - long load time for .wmv

1) why won't qt play .mpg files when VLC plays them with no problems?

While the MPG file extension is normally associated with multiplexed MPEG-1 video content on a Mac, this is not true for other platform users. For instance, the file may alternately contain anything from a simple MP3 audio stream to multiplexed MPEG-2 video or even AVCHD with AC3 audio. So in order to answer your question specifically, you need to tell us what specific form of compressed data is contained within the file. Basically, VLC provides its own built-in codec support which does not rely on your Mac component configuration while basic Mac support is limited to "playback only" compatibility of "muxed" MPEG-1 video with layer layer 1 or layer 2 audio unless you purchase/install specific additional component support like the Apple MPEG-2 Playback video only component and even then you would have to use a secondary application like MPEG Streamclip to to handle/bridge the synchronization of several audio formats.

2) why does it take sooooo long for qt to load a .wmv file (I assume it's using flip4mac) when VLC loads it almost instantaneously?

VLC plays the WMV data directly "on the fly" while the Flip4Mac component must read the entire file and cache the partially decoded data for direct playback by the QT player. You can avoid this delay by simply saving the partially expanded file data to an MOV file container using the "Save/Save As..." option. The resulting MOV file, however, will be much larger than the original WMV file.

Have you thought about installing Perian? It's a great codec pack that might help with your Mpeg problem. AND IT'S FREE!

Yep. That's the problem. Neither MPEG-2 video nor AC3 audio are natively QT compatible. You can purchase the Apple QT MPEG-2 video playback component at the Apple Store for $19.99 but QT does not have a core audio passthrough capability like VLC. Personally, if the audio is DD5.1 and you have a decoder attached to your computer optical output, I would stick with VLC playback here also unless your want to convert the file for TV 6-ch playback. (I use HandBrake to create H.264/AAC(stereo)/AC3(DD5.1) files that are playback compatible both with computer and TV devices but most users tend to prefer a non-conversion work flow.) Otherwise, the easiest playback options usually auto-convert the 5.1 audio to stereo for system level playback on most Mac platforms.