SpeedGrade render not using GPU, slow render speeds

Question

atHi!
I'm grading a fan-made feature-length film situated in this really obscure place called Middle Earth, and I'm currently rendering the final output.
Rendering speed has been really slow all along, so I decided to check upon GPU usage with a nVidiaInspector. It shows that GPU usage is between 0-5%. I'm able to get GPU usage to spike by switching to Monitor view (and scrubbing in timeline) instead of Output view in SpeedGrade. I understand that SpeedGrade is supposed to be GPU-accelerated throughout (not counting frame decompression made by codecs?), and now it seems that my CPU is doing all the lifting. CPU usage is around 100% all the time.
Render speed varies between 0.7 FPS and 6 FPS, which is far less than I expected, as I got reasonably fluid real-time previews all along the timeline when grading. Most typical clips have two primaries, two secondaries and a vingette mask primary on top. Total render time for 1 h 50 min timeline is expected around 12 to 18 hours.
My source material is 25 FPS 1080p CineForm AVI.
My output format is CineForm in Quicktime container (as AVI output with CineForm coded is not supported at present time) with automatic color calibration on, no audio, alpha discarded. Framing options are not in use (Full image (no proxy), 1:1 pixel aspect ratio, same size as proxy), and rendering is happeing  at Online Quality.
My machine's specs are:
- Intel i5 2400S quad-core
- 16 GB of DDR3 RAM
- nVidia MSI GeForce GTX 470 with 1280 MB memory
- Windows 7, 64-bit
- SpeedGrade 6.0.4x25
- GeForce 310.70 drivers
Any input is appreciated.

Answer 1

Maybe due to cineform quicktime?  Try other codecs to see what render speed you get. 
Regards,
Mark.
PS I am geetin slow renders to cineform quicktime files from other app's (Premiere, Media encoder...)