Outlook 2024 slows down massively when using remote credential guard

Question

I would like to share some information here that might save one or the other admin who is in charge of Office and/or network security a few gray hairs. Also, I hope some MS office developers read this and ask themselves how this is possible and fix it!

Namely: after switching from Office 2016 to Office 2024 LTSC (on premises Office, on premises Exchange Server), I noticed that the start of Outlook took a very long time (up to 15 seconds) and switching from one email to the next could take up to a sporty 10 seconds, depending on the size of the attachments.

After a thorough analysis, it became clear that this extreme only occurs if you use Outlook in a remote session AND also have the security feature Remote Credential Guard ("RCG") activated. I had connected to the office computer via RDP from the home office with a PC with active RCG via VPN, and that made the Outlook started on the office computer almost unusable in the new version.

So it is clearly the case that Outlook 2024 LTSC constantly (when any mail is previewed or opened!) passes the user credentials to the server (for whatever reason), while previous versions probably did not. If you are now connected via WAN and have passed your credentials through RCG, this is a massive problem, as every credential request costs time due to high WAN latencies. After I deactivated RCG, everything was immediately as fast as before.

For anyone who has this problem, it is recommended to disable RCG on the endpoints that are connected via WAN.

Versions, for the sake of completeness:

Client OS: Win11 23 H2, Build 10.0.22631.4602

Outlook version: 2408 (17932.20190)

Exchange Server: 2019 CU 14

PS: don't confuse RCG with normal credential guard ("CG"). This is really only about RCG.