The problem just got fixed after installing a Dell supplied update to the Intel UHD graphics driver. Now I remember installing another Intel UHD update about two weeks ago...Just tested a dozen sleep/wake cycles and all normal again.Beware of Intel UHD graphic drivers.  Francis
Greetings, System Config : Dell XPS8930 , Core i7-8700, Geforce GTX 1070, 16 Gb ram, 256 Gb SSD, 4 Tb WD hard drive. I keep Windows (20H02 ) and drivers updated as available in Windows Update and Dell Support Assist except for the Nvidia display driver which I keep updated to the latest version.  For many years when not using the PC I put it into sleep mode ( S3 ) whereby disks and fans come to a halt and only minimal system components remain powered on allowing for a swift resume. For good measure a full reboot is executed every 5-7 days, a habit from the Windows 7 days when the system became more and more unstable after long sleep periods. No significant new hardware, software or config changes were performed recently especially not in the power configuration settings. Since a week now the sleep mode ( this is disks and fans becoming silent and power on led pulsing slowly ) is delayed by 3 to 5 minutes. Both displays go dark right away after activating sleep and usually the system became silent a few seconds thereafter. Once the screens go dark I can not regain control, so troubleshooting is not possible at that point. It seems that the normal (fast ) sleep still succeeds, maybe the one just after a reboot, but this I have to verify. Any suggestions ? I have to try safe mode to check if the problem persists, but this will not really help because finding the culprit amongst dozens of drivers and programs that do not run is safe mode is almost impossible.   
XPS 8930 Delayed sleep