This is a long shot, but have you tried disconnecting the battery for a few minutes? My son's XPS 13 got into a situation where it was permanently throttled at

This is a long shot, but have you tried disconnecting the battery for a few minutes? My son's XPS 13 got into a situation where it was permanently throttled at 400MHz, and a random angel on the internet had tracked it down to a hardware safety that had triggered and could be reset only by disconnecting the power to let some low-level thing clear itself out. Since you have a newer model, they might have BIOS-level throttling that the OS knows how to override, which would mean that booting would be super slow. Worth a try -- good luck.