Enhancement: A performance test suite

We would benefit from a test suite that specifically checked on performance and looked for regressions.

There are some libraries around that specifically, like JSLitmus:
https://code.google.com/p/jslitmus/

See related issue for more background:
https://github.com/Khan/structuredjs/issues/14
(But we've also had other changes that introduced performance changes)

Start by reading related issue #14 and reviewing the JSLitmus project linked in the issue. Identify how a performance suite would fit this repository and which existing behaviors need coverage. Done means the suite can detect meaningful performance regressions, including changes beyond the case discussed in #14.

Written by the indexing model from the issue text.