JavaScript timing calculations could be more precise

All of the timing calculations appear to only leverage `Date.now()`... It would be better to use User Timing and/or Resource Timing if the user's browser supports it, or High Resolution Timing if neither is supported. Reason being is if a system clock adjustment happens to occur when doing the timing, the results will be inaccurate. Plus those other APIs provide sub-millisecond timing data.

Only if High Resolution timing is not supported should you fall back to `Date.now()`.

[Surf-N-Perf](http://comcast.github.io/Surf-N-Perf/) can help streamline that for you if you want to simplify your code, and that supports older browsers that don't support `Date.now()` as well.