Possibly related, but we were seeing some crashes with Present and introduced a couple changes in 5.6 to fix those which seemed to address the issue we were seeing, but this is a crash fix not a stall fix.
CL#38629989 (d3f049e) Relaxed presentation check with additional logging
CL#39799334 (740624c) Re-enable D3D12 dummy backbuffer to work around crashes due to exiting or re-entering exclusive fullscreen where the backbuffer index gets out of sync.
Do you see any RHI warnings or errors in the logs? If you’re able to reproduce the problem reliably it’d be interesting to see what -d3ddebug outputs.