Actually, ARM driver run at highest speed all the time. About a week ago I also increased lima frequency for smoother GUI experience.
Anyway, I doubt lima is at fault here. You know that many packages got updated from the time GPU driver switch happened, including Kodi. You can still build image with ARM driver although kernel driver might need update. Note that during video playback GPU normally doesn't do much, if anything. You can also run perf top to see which function takes most of CPU time.
chewitt lima will support DVFS with 5.8 and at least H3 has DVFS table already merged.
For the time being I have reverted to nightly-20200201-a869d6c build where CPU usage is 3% at idle and close to 60% during video playback.
I understand that things other than the GPU driver changed so if anyone else is bothered by this I will try to build the current code with the ARM driver to see if that really is the issue.
UPDATE: I tried to build but compiling the driver failed. As you said, it probably needs to patched for kernel 5.6 and that is over my head.