User Timing API https://w3c.github.io/user-timing/#dom-performance-mark

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This approach can be affected on subsequent reloads by V8’s preparser. This could be worked around by appending a random string to the end of the script, something Nolan does in his optimize-js benchmarks.

More context: https://medium.com/@addyosmani/javascript-start-up-performance-69200f43b201#.svmlrcyv7

KhoaNguyen 10-02-2017