Why is PP struggling to handle multi-camera sequence on my beastly editing workstation?

I've been using PP for years and I've never seen it struggle like this before.  I'm getting like 15 frames-per-minute playback on my timeline.  When I press play it takes 3 seconds to begin playing.  When I press stop it takes 10 seconds to stop playing.  It's impossible to accurately edit with these delays.

It's a 6 camera multi-cam sequence and I have playback quality set to 1/4th.  The original files are UHD 60p MP4 but I'm using the proxy media workflow with the 1280x720 60p ProRes422 codec.  The proxy media is on a NVMe SSD drive with a 3500MB/s read speed.  My PP cache is on a different NVMe SSD drive with equally fast speeds.  My PC has a Ryzen 3950x with 64GB of 3600MHz RAM and a Nvidia 2080Ti.  I'm running PP ver. 14.7

Can anyone tell me why my system is struggling to handle such a simple multi-cam sequence?