Proxy Files slower to scrub than originals

I'm at wits end as to what else to try, so though I'd see if anyone else has solved the same issue. I've been editing a video podcast for about 4 years now, so my workflow is pretty set - I have three 1920x1080 video feeds (24fps, H.264) displayed on screen at once for the three hosts, frames overlaid on top, other visual elements, etc.

My PC was a beast in it's heydey, but it's a bit old now (i9-7900X, GTX1080Ti, 64GB RAM), so jumping to a given point in the timeline always takes about 1-2 seconds to display the correct frames, and real-time scrubbing is impossible. No worries - I just create proxy files for the three videos, and voila - real-time scrubbing! Until a few months ago... no idea what changed (nothing new installed on my PC in ages), but proxy files now take longer to refresh than the original files! With proxies toggled on, it takes anywhere from 15-20 seconds to update! Turn the proxies off again, back to 1-2 seconds - what's going on?

The original files are about 3GB each; I've created custom Encode & Ingest presets that use Cineform YUV-10 bit, quality 1, 480x270 @ 24fps, and put the proxy files next to the originals (which sit on NVMe storage). This preset has always worked until recently, but in trying to troubleshoot this I've also tested ProRes 422 Proxy, hard-coded the frame rate, field order and aspect ratio (to avoid any confusion by ME), excluded audio entirely (I use a WAV file instead anyway), used the built in proxy presets, ensured I've followed all the best practices for scratch disks, cleared caches, everything I can think of! 

Any ideas? I just don't get why everything worked fine for well over a year, and suddenly stopped working recently /shrug