Why are rangefinders likely to provide an extra stop of hand-holding speeds?
Rangefinders don’t have a moving mirror, so they produce less vibration in taking photographs. This is one major reason that users claim to be able to get consistently one stop slower shots with similar lenses for equal results. In other words, if you can shoot and get good images at 1/30th with a 35mm f/2 lens on your SLR, you might get similar images at 1/15th with a 35mm f/2 lens on your rangefinder. That is very handy in low light situations! It would take an f/1.4 lens at 1/30th on your SLR to equal an f/2 lens at 1/15th on your rangefinder. In effect, you can squeeze an extra stop of performance out of your rangefinder in low light situations when shooting handheld (typically).