I was able to do some testing and it seems like you were spot on with the problem being related to my configuration somehow.
Here's what I was able to accomplish:
To install 12.7, I had to uninstall 12.9.2 which effectively cleaned out the configuration. I popped open DO and backed up my configuration prior to uninstalling 12.9.2, as you instructed, then uninstalled, rebooted and reinstalled but I started with version 12.8 instead of 12.7 because I wanted to see if I ran into the same problem with that version (never really got back to 12.7 in fact).
Upon initial installation everything was running great (at this point I was suspecting something with 12.9.x): windows were opening with prior speed and file operations were running great, etc; there was no degradation in performance at all even after multiple file operations. So I was going to uninstall and install 12.7 to see if the same happened there but I decided to restore the configuration to see if that would have any affect (again as you instructed) and, low and behold, it did: After the restore, things worked great for about ten minutes until I started working with files and then the performance started dropping and eventually slowed to a crawl again. Once the performance degradation started, even restarting DO didn't fix it: I restarted DO three times (exited DO and killed the DO helper application) but no-go; when I fired DO up again performance was still at a crawl. Before I uninstalled and reinstalled again, I rebooted to see if that would fix the performance issue and it did temporarily but once I started working files again, performance started degrading again. I'm not sure why killing the program doesn't fix the problem and a reboot does (memory issue maybe?) because when DO is closed, the tangential effects (slow window renders in other applications, file selects, etc) clears up but when the program restarts, the tangential effects are clear with DO still being sluggish until I start working with files and things degrade outside the application again.
After rebooting, I uninstalled v12.8 and reinstalled it to perform my tests again to make sure that I hadn't done something to mess things up. Tests went exactly as before: before I restored the backed up configuration, things were working great, restored the backed up config and things started crawling again.
At this point, I uninstalled 12.8 and installed 12.9 (not the beta, the GA release). Once 12.9 was installed, I repeated my tests and got very similar results. I say similar because there is still a performance drop on .9 without the restored backup although that seems to be related to EaseUS Todo Backup (when backup jobs are running is when I see the hit) but it's acceptable and I expect some performance expense with large file copies from the DriveBender pool since amalgamating 36 separate hard drives into a single virtual drive comes with overhead (and truly, that hit may have been there with .8 as well because I didn't test backups with that version of the software installed). That being said, once I restored the backup to 12.9, performance immediately dropped into the unacceptable range: switching lister panels was again taking 1 to 2 second and highlighting a stream of files was taking ten and required multiple lasso swipes which would sometimes create aberrant shortcuts, etc. Once I uninstalled and reinstalled without the restore, things seem to be back to normal; been using DO for the last eighteen hours with no hits in performance aside from the backup job degradation.
So long story short (too late), it seems there was a problem with my previous configuration although I do not know what as I have no network drives to speak of and to be honest, restoring the backup config didn't change anything on the surface: everything looks exactly like the default listers save for column placement. That's not to say it isn't something under the hood, but truly, there was very little visually that changed from the default config to the restored config. I'm going to email the backup of my config file, to the address you provided above, for analysis. I really would like to know what the cause of the issue is and hopefully it'll be able to help someone else who may run into a similar problem.
Thank you again for the guidance and for getting back to me in such short order.