Total Blocking Time

Hello Bilal,

Thank you for reaching out and I am happy to help!
TBT measures the total amount of time that a page is blocked from responding to user input, such as mouse clicks, screen taps, or keyboard presses. The sum is calculated by adding the blocking portion of all long tasks between First Contentful Paint and Time to Interactive. Any task that executes for more than 50 ms is a long task. The amount of time after 50 ms is the blocking portion. For example, if Lighthouse detects a 70 ms long task, the blocking portion would be 20 ms.
Eliminating render-blocking resources will yield the biggest improvement and in the W3 Total Cache you need to Minify the JS and CSS, make sure to select eliminate render-blocking CSS and deffer minified JS.
Please check the documentation below for more details:

W3 Total Cache: Minify Cache Settings Guide

Eliminate Render-Blocking CSS with W3 Total Cache Pro

I hope this helps!

Thanks!