hm its seems that wmvideo decoder dmo has the same problem as ffdshow with libavcodec, both dont work properly with interlaced vc-1 video as it seems.
in case of wmvideo decoder dmo its uneven distribution of the use of both cores. while its ~50 : 50 for every other progressive VC-1 video (hd dvd or blu-ray streams for example), its something like 66 - 33 or 75 - 25 in case with interlaced vc-1 video (example: galapagos HD DVD 1080i; sample:
http://www.sendspace.com/file/lvlvtv )