Hi Kirk
Sorry for the delay in getting back to you on this. I've finally had a chance to do a bit of testing.
So I can report that it seems that if the folder is stored on my local drive which is an SSD, then the tagging is instant. I've only been able to test this with a 25Gb folder with around 600 images, Raw + Jpg Medium. I had the identical folder on a Rugged USB C HDD and a San Disk USB 3 SSD and both had a lag of around a second when tagging. It does seem that after a period of around 5 minutes the lag sometimes disappears, but it's still nowhere the speed of being on the internal drive. If I use a bigger sized folder on either drive the lag seems more prominent. Could it be something to do with my drive settings, something windows based with external drives or PM's cache? Both drives are Fat32 as I use them on my Mac too, which as stated is instant and as quick as the windows internal drive. Both always seem fine with normal file browsing in explorer etc.
Also, even just switching to this browser to write this reply and then back to PM, when looking at the folders on the drive, the green sort bar is visible and then PM hangs and displays not responding for about 30 seconds before allowing me to continue. If I have the internally stored folder open this doesn't happen. If I could copy every wedding directly onto the surface internal SSD for culling then I would but I don't have the disk space, so some guidance would be greatly appreciated.
Another observation is when the tag/colour class lag happens, if I hit the arrow key to advance multiple times before the class is applied, it jumps forward by the number of arrow presses, as if it's happening in the background and then catches up once the class writing is complete.
I've attached screenshots of the settings you requested plus a cache settings shot.