Actually, in #401 @xaviershay has some perf improvements that this PR should take advantage of, I think: he's introduced the concept of a "simple stub" for these sorts of message/return-value pairs, and it looks to greatly improve perf.

So, I'm thinking we should merge his PR first (it sounds like he'll address my feedback soon and it's very close to being ready to merge) and then refactor this to use `add_simple_stub` under the covers.

Thoughts?