I'm sure faster times are out there but no one has posted them yet. So I figured I'd start the conversation. I'll throw out Nathan Morgan and my attempt at an FKT for Cloud Peak from the West Tensleep Trailhead. Round trip time was 6:05:47 (took the slowest of the 3 times posted, there's only 10 seconds difference between the 3). Started at 6:23AM on July 9th, 2017. Nathan reached the summit in 3:15:10 and I reached the summit in 3:21:40.
My movescount data
My Strava data
Nathan's Strava data
[Note that summitpost.org indicates the ascent has been done under 3 hours, but we have no details.]