TTI is (currently) a Chrome-specific measurement that measures the time until the page being loaded is considered usable and will respond to user input.
TTI is built on top of a collection of other measurements, some of which are currently only available in Chrome:
The measurement is of the first paint after the layout where there was the most change in layout in the visible part of the page. It is meant to measure the point at which the main content of the page is displayed and is a placeholder until a better method for sites to indicate their main content is available.
Chrome exposes this measurement as a "blink.user_timing" trace event with a name of "firstMeaningfulPaint".
The browser's main thread is considered "interactive" when it is not blocked for more than 50ms by any single task so it will be able to respond to user input within 50ms. An interactive window is a period of at least 5 seconds where there are no main-thread tasks that take more than 50ms.
At any point in time this is the number of outstanding requests.
- Start looking for TTI at the first meaningful paint
- Look for the first interactive window where there is a contiguous period of 5 seconds fully contained within the interactive window with no more than 2 in-flight requests
- TTI is the start of the interactive window from step 2 or the first meaningful paint, whichever is later