I imagine few emulsion makers will consider using cadmium given its extreme toxicity for minimal reward. I actually have a bottle of it, but I think I'll finally get around to disposing of it when the local sanitation company has its next HazMat collection day.

None the less, the discussion is interesting from a historic perspective. I dug through Glafkides (Photographic Chemistry, Vol 1, 1957) this morning.
"Cadmium is a retarder of physical ripening. Its action with chloride emulsions results in an increase in contrast. The addition of cadmium salts is made before precipitation." p.318.

Gladkides then goes on to spell out a number of chloride emulsions with increasing levels of cadmium. An "Extra hard unwashed chlorobromide emulsion" has 80 g NaCl, 20 g KBr, and 40 g of cadmium chloride for 200 g of silver nitrate. (It also has sulfuric acid, egg albumen, and "color sensitizer".) It's the kind of recipe that gets geeky hearts all in a twitter. But, I think, best to leave that one safely on the page -- not running loose in the home darkroom.