Web development and performance
What is Total Blocking Time?
Total Blocking Time sums the portion of every long task over 50 milliseconds between First Contentful Paint and interactivity. It is a lab metric and the closest available proxy for the field metric Interaction to Next Paint.
Also called: TBT
How to read it
Under 200ms in a lab test is good. Above 600ms means the page is unresponsive during load, and a user who taps early will feel it as the interface ignoring them.
Why it is a proxy rather than the thing itself
TBT measures blocking during load. INP measures actual response to actual interactions across the whole visit. A page can have acceptable TBT and poor INP if the expensive work happens on interaction rather than on load.
The usual causes, in order
- Framework hydration on a page that did not need a framework
- Third-party tags, especially tag managers loading further scripts
- Large bundles parsed and executed before anything else can happen
- Animation libraries loaded for a single effect
Where this is covered in depth
A definition can only go so far. Core Web Vitals, and the order that actually reduces them covers this properly — 3 minutes, free, no email required.
Who wrote this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.