I started using a rangefinder in college and began my career with one. One of the most irritating things I hear is how Rangefinders force you to slow down. I can react and focus and shoot just as fast with a Leica as I do with an SLR. It comes down to experience. I think the whole "slowing down" is due to the unfamiliarity. If you don't know how to use something of course it's going to slow you down but rangefinders are not slow cameras.
If anything, rangefinders are quicker than DSLRs - and film SLRs too, for that matter.
If you use hyperfocal and/or zone focusing techniques, you have no "focus slowness" for lack of a better term. Then there's the issue of autofocus - with a manual focus rangefinder, there is no delay while the camera hunts for proper focus. Then there is the issue of shutter lag. This is not so much of an issue with film cameras, but it is an issue with DSLRs.
I did some spec comparing on shutter delay times of DSLRs vs. Leica M film rangefinders. It has been awhile since I did this so I do not recall which models of cameras I researched, but I do recall this: The shutter delay time of the Leica M cameras was 1/8 that of the DSLRs I researched.
With a manual focus rangefinder and zone focus, the limiting factor is not autofocus time or shutter lag; it is how quickly you can move the tip of your right index finger.
The shutter lag times in milliseconds for a few well known cameras are as follows -
Sony NEX-5: 115ms
Sony Alpha DSLR A850: 74ms
Nikon D300s: 53ms
Canon EOS-1D mk. IV: 49ms
Canon EOS-1D mk. II: 40ms
Nikon F6: 37ms
Leica M3: 16ms
Leica M7: 12ms
(source:
http://en.wikipedia.org/wiki/Shutter_lag#Examples_of_various_shutter_lag_times )
Quicker is better in street photography. "Nerding out" by comparing shutter lag times that are measured in milliseconds may seem silly or pointless to some.
BUT - when you add shutter lag + autofocus hunting time + photographer reaction time, it can all add up to the difference between getting the shot or missing the shot.
In addition to the above, rangefinders are
Smaller and lighter than DSLRs
The lenses are smaller and lighter
The lenses have faster maximum apertures than all but the costly, heavy professional DSLR lenses
The shutters of rangefinders are much more quiet than those of DSLRs and film SLRs
There is no whine of the film advance/rewind motor as there is with a film SLR
There is no unholy
THWACK/WHEEZE racket when you actuate the shutter as is produced by Hasselblad V system film bodies
The lenses are far more sturdy and reliable than autofocus zoom lenses
The lenses are miles ahead of DSLR lenses in terms of image quality (Leica M and Zeiss glass, at least)
EDIT
...In a rangefinder, you look at the world, and you put a frame around what you see, in a SLR, you look into a black tunnel and you try to see the world.
Very true - a rangefinder gives the photographer a totally different way of seeing
their subject - and the world - compared to a SLR. You have a set of framelines and a non-framed zone outside of the photograph so you can see when a subject is about to enter your frame. This gives you a huge advantage in terms of anticipation of subject movement outside the frame compared to a SLR.