Traditionally, that was indeed the speed ranking of the Slams. This held firm from the time the Australian Open moved to rebound ace in 1988, right through the 1990s.
In 2000, the tournament organizers gave the AO courts a fresh lick of paint, and the court was much faster that year. As the new Ashe stadium in New York was also (at the time) very fast, and much faster than the old Armstrong (it's the reverse today), in 2000, there were three Slams on very fast courts. I'd say it was almost RG > AO=UO=WI.
In 2001, the AO courts were a little slower again, and by 2002 just as slow as they had been in 1997, 1998, and 1999.
In 2002, the new grass at Wimbledon was much slower, and you're right that for some years the UO may have been faster.
But starting in 2011, there's been more sand in the UO mixture, and it's gone from being very fast for a hard court to extremely slow.
Right now, I'd say it's RG > UO = AO > WI.