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I think the point Terry was trying to make is that as far as the brightness of the image in the eyepiece is concerned, fast and slow are not important. Yes it can affect the degree of coma and the length of the optical assembly and hence the ease of use, but that is a function of its focal length rather than it's speed. We still often refer to a scope as being fast or slow even when used visually as it is a simple shorthand way of describing it.
BTW Raymo in his post mentioned that longer focal length scopes will suffer less coma or chromatic aberration. That depends on the type of scope. For a newtonian (and dobs are a form of newtonian) only coma is affected as they do not suffer from chromatic aberrations.
Malcolm
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