I added a benchmark file including the output. Turns out at 1k invocations passing as a proc is always faster (usually around 2x faster), at 5k it's usually slower (measured in sub-millisecond diff per 5k invocations), and at 10k it's always slower.

So the question, in terms of performance, is how many times does this get invoked in a suite. The answer is 1 time per around hook per example. In most suites that number is zero. In rspec-core we have one global around hook and a little over 1k examples, so it's 2x as fast (again, we're measuring in sub-millisecond values) to pass it as `&proc`.

I think this evidence suggests that (not proves, as it's not taking other forces like GC into account), from a performance perspective, `&proc` is the right choice here.

The other question is what story does it tell. I've always preferred to pass procs around with an `&` because it clarifies what the thing is regardless of the name.

Please feel free to poke holes in the research (i.e. the benchmark is flawed in some way, etc) or point me to different evidence (empirical or anectodal).