When total blocking time is reported, it does not seem to match the data in the table provided. Attaching two screen shots.
One says Total Blocking Time = 545
and lists these five times in the table: 284,279,74,58,12
On another test, the Total Blocking Time is much higher, yet the individual times in the table are lower?
Total Blocking Time = 975
five items in the table: 251,204,103,39,4
How do I understand this apparent discrepancy?
We need to make this clearer, for sure. It’s not just you.
The Total Blocking Time that is reported in most places in the UI (545 in the example above) is the Total Blocking Time for the page, calculated by looking at all blocking tasks that occur between First Contentful Paint and Time to Interactive.
The Main Thread Blocking Time in the table is just the total blocking time for JavaScript related work.
So, typically, we see the Total Blocking Time as higher than the total Main Thread Blocking Time since Total Blocking Time would include long tasks for styles, layout, etc and the table only looks at JavaScript.
In the case where you see Total Blocking Time as lower than the total in the table, that’s because there are some long tasks that are occurring before First Contentful Paint. Right now, that gets totaled in the Main Thread Blocking Time table, but those tasks wouldn’t be included in the Total Blocking Time metric.
Clear as mud, huh?
We’ll work on making this a lot more consistent and clear in the UI.
This helps a lot…thank you! And thank you for all the value the WPT tool provides.
