Okay then... You're using the same version as I am. So you're seeing lockups of ~5 minutes per file that get's thumbnailed? That seems quite extreme. I am not getting lockups quite to that degree, but I do sporadically have some performance issues when Thumbs are building however -- and we are not alone (and I believe they are working on it).
I generally keep the "Build Thumbnails" option de-selected when I run imports, and then let MC build them on the fly (and occasionally just build them manually to get "caught up"). I do sporadically get long 3-5 minute pauses while browsing around in my videos, and I am using CCCP. Some similar issues other users have mentioned have had to do with Podcasting Auto-Downloading, which is why Jim mentioned it. Could you do me a favor and, even if you don't use Podcasts, go in to the Podcasts item in the Tree and click the Disable Auto Download button to make sure it's turned off. Its interesting that building the thumbnails on import is seems to be getting stuck on certain individual files...
If you switch only MPEG-4 ASP (DivX/XviD) from FFDSHOW to the DivX filter then all the playback issues go away, correct?
Do you have other video file types that decode using FFDSHOW? Any MPEG-4 AVC stuff (Quicktime H264 MP4s, Nero Digital AVC, X264 videos)?
Also, if you could generate a log file that includes this behavior (but not a whole bunch of extraneous activity -- so close MC, open it, reproduce the behavior, and then grab your logfile and save it), I suspect that would help them. I'm going to try to do the same thing for them, but I have a bunch of network files and it helps if you can eliminate those possibilities. Sounds like we might be able to give them a couple of similar reports to investigate!