Hi Marc, I'm experiencing a pretty similar situation, with Windows 8.1 PCs hard wired at 1Gb/s to a Synology NAS.
I open a directory per contact sheet, up to about 20 simultaneously, each containig 1000 to 3000 images (RAW + JPG). The overall disk access speed abruptly slowed down in the last month or so, and the load on the CPU in the NAS stays at 90-99% for minutes, either after opening a new contact sheet or opening or saving a file.
I'm afraid that the problem is not due to Photo Mechanic, even if it gives an heavy contribution to it.
Have you someway solved your problem? How?
If not, if I find a solution I'll let you know.
Roberto