Windows Server 2025 over SMB much slower than 2012

Question

Hello,

we used to have a Windows Server 2012 running on VMWare that has been used as a SMB file server. The (virtual) harddisk is about 6 TB. Access based enumeration was used for the SMB share on this hard disk.

To migrate this server we have set up a new Windows Server 2025 installation and moved the harddisk from the Server 2012 VM to the new Server 2025 VM within VMWare.

We did migrate the SMB share configuration via Registry Export / Import.

Now we realize that when we access directory shares via SMB on the new 2025 Server it is much slower than before, especially when browsing large directories with more than 2000 - 3000 directories or files. But this is only true when doing this as non-admin, as an user with admin rights the whole directories are listed immediately even when they are large with more than 50.000 entries. Also when browsing the directories locally on the server its fast - the problem only occurs via SMB.

It seems to me that this is kind of a typical behaviour when having ABE activated and i can also confirm that when i create new shares with ABE off and put a few thousand files in it, browsing those directories over SMB is very fast, when i activate ABE it becomes slow (both is only true for "normal" users, as Admin User its always fast)

But what i dont understand is why it is so much slower on Server 2025 compared to 2012. We didn't use the ABELevel parameter on the 2012 server and the (virtual) hardware for the 2025 server is much more powerful than it was for the 2012 server.

Can anyone give me a hint how to improve this without turning off ABE completly?

Answer 1

That's an excellent observation, and you've perfectly diagnosed the core issue: Access-Based Enumeration (ABE) is the culprit.

To improve performance without disabling ABE, I recommend two steps:

Enable Directory Cache (DirCache): This is the most effective setting for this scenario. You can configure it via PowerShell: Set-SmbServerConfiguration -DirectoryCacheLifetime 30 -Force. This tells the server to cache directory listings longer, dramatically reducing repeated permission checks.

Verify the "ABELevel" parameter: Even though you didn't use it before, explicitly setting it on Server 2025 can help. Try Set-SmbServerConfiguration -Abalevel 1 -Force to use the newer, potentially more efficient implementation.

Start with the DirCache setting, as it's most likely to give you an immediate performance boost for your users.

Hope this gets your file server performance back on track. If this resolves the issue, please hit "Accept Answer" 😊.