WebPagetest/AFT Usage Notes

Life's too short for slow pages

1) For best results, after_load_delay should be five times as long as the longest page load time you would expect (at least two minutes). Animation on the page is identified and ignored if (a) the screen changes often, (b) the screen changes unusually late in the load. Both of these identification mechanisms will be much more successful if the screenshots span a longer time period after the page is loaded.

2) The status message indicates how confident you can be about the returned AFT measure. As long as early_cutoff is set sufficiently high (should be larger than it takes any page to load), a "high confidence" measure signifies with 99% confidence AFT is the moment the page stabilizes (earliest point in the load after which only repetitive/contained animation occurs).

3) To conserve resources and time, the screenshots can be run for shorter periods and rerun for progressively longer periods if the status message is not 'high confidence'.

4) If only a short timespan of screenshots is available, the parameters can be set to return useful metrics other than AFT such as 'latest first render' of the page.