Why is iMovie export to using so little
I'm using iMovie 10.1.11 to export 25 minutes of 720p H.264 to a 720p file,
running on a 2018 MacMini 3.2 GHz Intel Core i7, with 32GB RAM, with Mojave 10.14.5,
and a Thunderbolt 3 connected Radeon RX 580 8GB eGPU.
iMovie is only using 3% of the CPU, 64 threads,
and the ProTrranscoderTool_sp is using 0.5% with 33 threads.
The GPU History shows the AMD Radeon RX 580 processing the video but only one dot high.
The export takes about 24 hours.
There are no transitions, no mixing of audio, for the first 20 minutes of the video it should be a straight container copy, and then the last 5 minutes have a closing titles.
Mac mini, macOS 10.14