Your situation is caused by processing DSD source material through a large convolution filter generated at a low sampling rate. In this situation, Roon is forced to upsample the filter by a large multiplier, creating an absolutely massive load on the convolution engine. A long filter generated at low PCM sampling rates is not a good match for the “Native DSD Processing” option, and would struggle on any CPU.
You are in this situation because you started with REW, and REW is incapable generating filters above 192kHz. REW’s authors were probably not thinking about DSD when baking in that limitation.
The latest beta of REW includes the ability to generate 352.8k and 384k filters. This could reduce the CPU load significantly, so it may be worth a shot.
@wklie’s suggestions above are good if you want this to work with your current filters.