Benchmarks in docs

[adam-coster]

Since there are built-in methods doing some of the stuff that SNAP does, and since in general it's useful to know what the performance of tools is going to be, it would be great to have some benchmarks in the docs. In particular head-to-heads against built-ins, where available.

[JujuAdams]

It's going to be slower than native in basically all situations, and likely substantially so.

[adam-coster]

Fair enough! Mostly I'm curious if it's slower enough to matter in most contexts.