Nice update to the support forum, by the way!
I understand you already know about this issue but just wanted to add my experience with it.
In my case, for at least several months now, if a network share that I have open in one of my tabs (not necessarily the current tab, and not even loaded) is temporarily missing, Directory Opus hangs 10++ seconds for every event such as switching tabs, switching listers, loading/refreshing lister contents, opening a folder...etc.
I already had Opus set as mentioned previously:
Preferences / Folder Tree / Options / Populate contents for local devices only.
Preferences / Folders / Auto-Loading / Network drives: Prevent loading.
and it definitely doesn't make a difference. This happens even when the shares weren't previously mapped as a drive letter, only open using the UNC path in a tab, but has happened whether the lister was loaded (from previously when the network share was available) or if it's unloaded.
The extremely slow Opus startup happens as well in some but not all of these cases, or at least to varying degrees perhaps dependant on how many tabs are open with the offending UNC paths, so much so that in some cases I give up on it loading and terminate it with Task Manager. I can definitely say that it's likely that I had many more tabs open to UNC paths which were temporarily invalid when I had the experience that the loading time of Opus was unbearable. I also definitely had mapped drive letters to the UNC paths as well when my worst experiences with the issue happened, but definitely didn't have mapped drive letters to the affected UNC paths in the "mildest" cases - only had them in tabs and unloaded.
I don't know when it started happening as having network shares go down more often is a relatively recent occurrence so I might not have noticed until several updates after it might've started happening. I can't even swear that it didn't happen on 11. I am on 12.3 stable right now and I regularly upgrade whenever a stable version is released.
This has almost been a feature rather than a bug as it was my first evidence that something was wrong with one of my network shares several times now, so if I could opt into a pop-up notification dialog of the fact instead of the other symptoms, that would be greaaaat. 
Only kinda kidding on that.