PDHanumanSvr.exe which generates the shadow files is CPU throttled in PD, documented by many on these forums. Your 20% is typical.
To my knowledge, use of any form of HA or the like won't help as the PDHanumanSvr.exe is canned to use only CPU encoding to generate the low res editing file so any user settings will do nothing.
I have not snooped around if the shadow file resolution can be tweaked via an ini file, regedit or the like as I don’t typically use them. I’ve used this approach successfully in the past with a previous PD version. I took my original footage, say orig.mts, and produced a very low res production, say lowres.mpg with PD. This is really what PD shadow file is doing but one can utilize the full CPU, not a throttled service or even HA to produce the low res editing file. I then edited the lowres.mpg file in PD and did my cuts, transitions and the like, just not screen snapshots for obvious reasons. When done editing I simply replaced the lowres.mpg file name with orig.mts in the *.pds file for my final production or you can rename files in the folder. One can even batch produce this pds file if desired. It worked fine for me, it does require that no frames are dropped for some reason while creating the lowres file compared to the orig file so match timeline fps in the lowres production.
I verified proof of concept with some downloaded 4K footage and it worked fine. It might be worth a try for you.
Jeff