Actually it's a bit shorter than an hour - tested it today. It took over 2h once when I did it for 80cd brightness and with both drift compensations enabled.
Yes, since drift compensation does a lot more black readings, it's going to slow things down a lot. You probably don't get much out of it for any of the instruments that use the light to frequency sensors though (i1display 2, huey, Spyders) or are temperature compensated (DTP92, 94). It's really intended for the spectrometers (ColorMunki, i1pro). Graeme Gill.