A few thoughts while MTUSupport ponders this problem.
1. Are your Hard Drive and CD both on the same cable or on separate cables. If on the same this could cause the slow down because the song you are playing and the one you are reading (importing) are sharing the same data path.
2. Are any other programs running in the background such as Nero, Roxio and antivirus program or spyware program. These will all eat up working memory. Have your computer guy shutdown any unneeded processes. When you are doing your show disable your antivirus and spyware programs, etc.
3. Just a wild shot but when was the last time you defragmented your hard drive? The more fragmented it gets the slower your machine goes.
You said this didn't start until you loaded version 3.300. I don't know of too many programmers these days who clean up their code. Most just jump over bad or changed code and leave it in the program. (Not saying MTU does that
) Version 3.300 did add quite a bit of changes and additions which means Hoster's resident memory size increased making the amount of free working memory more critical. So what may work fine with earlier versions may start causing problems with newer/larger versions.