Avisynth/ffmpeg cpu usage 30% or lower

[Mawazi]

I'm trying to figure out why avisynth/ffmpeg are not using more cpu time. According to task manager, I'm barely getting up to 30%. Memory is fine, about 20% in use. I'm using multithreaded avisynth, with what should be a fairly intense avisynth script that uses qtgmc:

LoadPlugin("C:\Program Files (x86)\AviSynth 2.5\plugins\DGDecode.dll")
SetMTMode(1,8)
video=MPEG2Source("C:\Users\Mark\Documents\DVDFab9\Video\BBCDVD3672\arkinspace4.d2v",idct=5,cpu=0,info=3)
audio=WAVsource("C:\Users\Mark\Documents\DVDFab9\Video\BBCDVD3672\arkinspace4.wav")
AudioDub(video, audio)
AssumeTFF()
# SetMTMode(2)
QTGMC(Preset="Placebo", SourceMatch=3, Lossless=2, EdiThreads=8)
Interframe(NewNum=60,NewDen=1,Cores=8)
SincResize(788, 576)
Crop(10,0,768,576)

I've tried a number of SetMTMode parameters without any further cpu usage. I've tried upping the cores/threads to 16, no more cpu usage.

I've used ffmpeg with just copy for vcodec and acodec, but get the same result as far as fps encoded, which makes it seem that ffmpeg is not at fault. Or is ffmpeg possibly not using all of the cores/threads?

Hardware wise I have an Intel Core i7 4790k, 16Gb Ram, and a Samsung 850 Pro SSD. This is a recent hw build.

It seems like there is a bottleneck somewhere that is obviously not cpu limited, but I just don't understand it.

Any ideas?