Total Blocking Time is a companion metric to Time to Interactive. For each task, it considers all time except for the first 50ms to be "blocking time". Total Blocking Time is, then, the sum of all the blocking time that fall between FCP and TTI.

As a matter of fact, the Time To Interactive does not measure how long it takes for a page to become interactive, it measures how long it takes to be sure, regarding the conditions, that a interactivity can happen in a satisfactory way, for at least 5 seconds.

"The result was a set of filters that whilst not vastly visually different, finally became interactive for more of our users as the loaded bundle was reduced to 53KB; a 70% reduction. Once gzipped that reduced further to 13KB."