I'm stubborn, so I had to figure this out myself.
Wireshark showed:
SMB2 131 Create Response, Error: STATUS_OBJECT_NAME_NOT_FOUND
SMB2 131 Create Response, Error: STATUS_FILE_IS_A_DIRECTORY
SMB2 131 GetInfo Response, Error: STATUS_ACCESS_DENIED
SMB2 166 Lease Break Notification
I thought it might be NTFS permissions, but even after re-applying security settings per DP's KB: https://wiki.covecube.com/StableBit_DrivePool_Q5510455 I still had issues.
The timer is 30 seconds, adding 5 seconds for the SMB handshake collapse. It's the oplock break via the Lease Break Ack Timer.
This MS KB helped: Cannot access shared files or folders on a drive in Windows Server 2012 or Windows Server 2012 R2
Per MS (above) to disable SMB2/3 leasing entirely, do this:
REG ADD HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\lanmanserver\parameters /v DisableLeasing /t REG_DWORD /d 1 /f
I didn't need to restart SMB2/3, the change was instant and file lookups and even a simple right-click in Explorer came up instantly. A process that took 8days+ finished in an hour or so
Glad to be rid of this problem. Leases are disabled yes, but SMB oplock's are still available.