Just want to leave here - my evaluation of VC6 performance (via GLES 3.1/ComputeShader from latest mesa 19.3-dev - see @jamesh post from 20191011) show good performance dependency from frequency only on few values: 500MHz(std), and downclocked 300MHz, 250MHz.
Values like 400, 600 and 700MHz don't show this dependency.
I believe, that VC6 synced with memory controller or some internal bus, but have no idea about values/steps. (and looks like not synced with ARM's cores). Or firmware-relative.
PS: looks like VC6 dependend from optimal workgroup size much more(almost linear between 100 and 256) than a JetsonNano's MaxwellGPU/nvidia drivers.