Oops, this is what I originally wanted to post: 
http://www.spitfireperformance.com/spit1vrs109e.html
RAE testing of the Emil vs Spit, scroll down to the Roll Rate section, theres a graph
Between 200-250 mph the Spit takes twice as long to bank 45 degrees!
1 second for the 109, 2 seconds for the Spit 
While relatively indicative of performance, time to bank is
not the same as steady state roll rate performance. You cant say the 109's steady state rate of roll is twice that of the Spitfire at 200 mph based off a time to bank 45 degrees.
There are all sorts of factors than can slow/aid the onset of a roll or contribute to a slow initial roll rate. Everything from the gearing of the control column, the type of aileron, type of aileron covering, tension/slack and stretch in the control wires, the state of airflow over the wing and aileron. I believe one of the great assets of the Fw 190s outstanding roll was that it used control rods instead of wires.
IIRC, there is a RAE/AFDU or NACA paper dealing with roll rate on the Spitfire that has some interesting graphs on the delay between control column movement, aileron movement and the actual initiation of the roll. Its been a while since I looked at it, but IIRC (again), there is up to 1/3rd of a second delay before roll onset at some speeds.