Depending on your computer power, it sounds like BIAB is overtaxing the CPU. It would help if you would tell us what you're using.

I've seen this when using RealTracks. Check RT settings and disable 'Speed up generation' and 'High Quality stretching...'. Also, if you're running other applications - that could cause it. Open your Windows Task Manager (Ctrl Alt Del) and see what is using CPU time while this is happening.

R