Will lens adapter ring with optic restrict light?

I have several M42 screw mount lenses, fast and old but still good. Of course they won't fit my new Nikon DSLR, so I was looking at the adapter rings, specifically the ones with the glass optic that allows the lens to still focus to infinity. (Otherwise, a simple adapter moves the lens away from the focusing area, preventing infinity.) Anyone have any experience with them? Does the ring's optic restrict the amount of light passing through when using a fast lens?

Not only the optic but the fact that you're extending the lens - yes… Perhaps as much as 2-stops.
I understand your reasoning in trying not to invest in new lenses but I think you're going to find that's what you'll have to eventually do if you hope for any real technical quality from your dslr.

Lens adapters really don't work very well - often causing focus and, as mentioned, f/stop problems.
Nikon cameras and associated lenses are designed to work best together.

If I were you I'd skip the entire adaptor idea and just buy a Nikon lens - you'll be much happier in
the long run.