Inconsistent CPU usage on export

Hello,

I'm trying different export scenarios for an HD XDCAM EX 1080i (HQ) 25fps sequence.

If I use the default PAL DV AVI or OP1a XDCAM EX 35mbps preset with CUDA accelerated MPE(Mercury playback engine) the CPU usage on export is 80%+ and the export is very fast.

If I use the default OP1a SD presets with CUDA accelerated MPE the export speed is very slow and CPU usage is below 10%.

If I use any other "custom" SD or HD AVI codec setting CPU usage on exports falls below 10% with CUDA accelerated MPE.

CPU usage stays at below 75% with software only rendering. Export with software only MPE is a bit slower than with hardware accelerated MPE with default presets.

Is there a particular reason for CPU usage to fall with custom settings?

Premiere is 2017.1.2

Media Encoder is 2017.1.2

The system is Lenovo Thinkpad W520

CPU: Intel Core i7 2760QM

GPU: Quadro 2000M

RAM: 8GB

[Moderator note: moved to appropriate forum]