@Nic
Thanks for the update. Here is a quick speed comparison between MPEGDecoder v2.03 and mpeg2dec3 v1.04 (idct=5). AVISynth 2.5.1 RC2, P4 2.4 GHz FSB 533, 512 MB PC266 DDR, simple script to frameserve PAL 720x576 video @ 25 fps with no filters except ConvertToYUY2, CCE 2.66.01.07.
mpeg2dec3 v1.04: 1.72
MPEGDecoder v2.03: 1.89
Maybe still a little room for improvement