The issue is the logic for claming `BandwidthDeltaTime` in `UNetConnection::Tick` to 1.0f / DesiredTickRate with DesiredTickRate typically at 120 Hz independent of a actual framerate.
A proposed fix can be found on UNetConnection: prevent unintended bandwidth throttling; clamp hitch window to 100 ms by NorbertNemec · Pull Request #13731 · EpicGames/UnrealEngine