We are running a lightly modified Horde server and agents based on 5.5.4-CL40574608 with this patch: https://github.com/EpicGames/UnrealEngine/pull/13089
The server host is not running any scanning software. IIRC the agents are running Sentinel One, but I don’t see the same slowdown when other apps are performing similar large IO writes.
I think I have it narrowed down to BatchOutputWriter having high internal contentions when the same file is split across many chunks sets from many different blobs. Will report back if I come back with any promising optimization results.