As I've worked on perf improvements or implementing new rules in the past, I've found it useful to be able to see where significant time is being spent within rules with greater specificity than the overall view given by npm run perf. Does anyone else think this would be useful functionality?

I decided to push up my local timing branch to show what I'm doing. If there's interest, I can put together a PR. Since aggregating timing data incurs performance penalty outside of the rules, it only runs when I enable it. For example, to see the ten longest-running rules on the ESLint source code:

$ TIMING=1 eslint lib
camelcase: 51ms (6.3%)
no-irregular-whitespace: 42ms (5.2%)
no-multi-spaces: 40ms (5.0%)
valid-jsdoc: 40ms (4.9%)
handle-callback-err: 30ms (3.7%)
no-undefined: 24ms (2.9%)
space-infix-ops: 23ms (2.9%)
no-caller: 22ms (2.8%)
no-empty-class: 21ms (2.6%)
semi: 18ms (2.2%)