With Wizards, I've pulled off a 52 thru 54 in 3 consecutive rifts and I believe the 52 was under 10 minutes. I then tried 2 55's and both were horrible rift layout with terrible progression mobs thus spectacular fails. Of course this was non-season with 730-ish paragon and I haven't attempted anything that high since. Once I'm out of speed farm keys I'll revisit the 50's and check average clear times. Also able to pump out 53-55 keys in trials consistently but I think I'm going to try a Tiki WD since it's the instant wipe from leaping mobs that end the trial sessions. Perhaps I can lock them down with fear before I'm beheaded.