I dunno, whats the fastest - and is the fastest to make the rank the best RSM? A number of good soldiers have gone half way up then down to the bottom again before bouncing into the RSM's chair. It must be hard for someone to be the scary man who has seen it all if they haventl spent enough time in each rank to see what is going on.
In wartime Frederick Hillerson "Ben" Keeling DCLI MM went from Private to WO2 in about a year. But he was a socialtist who refused a commisison on political grounds and killed at Delville wood in 1916.