I had the same problem with vice city when I transferred music using Virtual CD Ripper.
Best thing to do is to rip tracks the normal way, it's long and boring but you should work on it with a few mates but only one of your xbox and after you've made a decent soundtrack, transfer to each others xbox.
Remember if you're planning on updating the soundtrack then it's best to do one xbox because if you do one album on one xbox and another album on another xbox, you won't be able to send just that one album because the sountrack information is stored in a database.
Sorry if you already know this but I thought it's best to explain it all just incase.
Other reasons I think it might slowing down is becuase of the bitrate and maybe also the WMA version, I think it's supposed to be version 8 not WMA version 9