I've just done some experiments with Vegas 8c on an 8-core machine with RAID, on getting HDV (1440x1080 25fps) to play as smoothly as possible in Preview. Surprisingly, my results indicate that it's best to just plonk the m2t files on the timeline. For my system at least, transcoding to Cineform NEO-HD (and setting decoder to Fast) or inputting the "m2t" via AviSynth just made it worse. Even if media were interlaced and project was progressive, transcoding to Cineform as progressive didn't help. Cineform and AviSynth are useful in other ways but not, in my case at least, for speed. Anyone agree/disagree with my results?

The results in more detail (for an 8-core machine with RAID, where as a guide, one core at 100%=1/8=13%, two cores at 100%=26%).

# The best results for “m2t” files are obtained when using Sony Vegas's internal m2t decoder and Preview is set to Half/Good. For both full-frame and zoomed-frame, this gives 25 fps at 14% CPU.
Neither substituting CFHD as input media nor decoding via AviSynth-DgMpegDec-VFAPI are able to improve upon this, in fact they make it worse.
# For full-frame:
* The best CFHD result is 25 fps at 25% CPU but only when the decoder is in Fast mode, otherwise only 18 fps at 25% CPU if it is in Quality mode.
* The best AviSynth-input result, occurring for Good/Quarter is 22 fps at 17% CPU. For the more likely-to=-be-used Good/Half it is 18 fps at 18% CPU.
# For zoomed-frame:
* The best CFHD result is 25 fps at 25% CPU, i.e. the same as for full-frame. The result for the more likely-to-be-used Good/Half is 24 fps at 30% CPU, regardless of whether the decoder is in Fast or Quality mode.
* The best AviSynth result is 22 fps at 17% CPU for Good/Quarter. For the more likey-to-be-used Good/Half it is 19 fps at 16%.