Using SOCKETS brought down the times a fair bit:
On the amd fx8150 where 6.1.1 takes 44+/-3 seconds, 6.3 now takes 56+/-1 seconds (10 repeats).
On the intel i5-2400 where 6.1.1 took 50 s, 6.3 now takes 70 s.
On the dual-socket xenon node 6.1.1 took 78 seconds (retested), while 6.3 now takes 107 seconds.
My times are now more in line with Vladimir's i.e. 30% above 6.1.1.