Hi there,
INP is based upon the time between a user interacting with the page, and the browser repainting the page to show the interaction occurred.
A good example: User Clicks Hamburger Toggle — time passes as browser does stuff –> Browser repaints page with Menu Open.
Note: Scrolling the page does NOT count as a user interaction.
Anything the browser has to do between the user interaction and repainting the screen is going to be the major factor here.
Measuring that can be tricky, you can record the performance of network activity to see what ‘events’ occur after an interaction.
The most likely culprit will be any optimizations that delaying the loading of scripts until the user interacts with the page.
NOTE: INP is still an experimental metric, therefore it won’t be a ranking factor, ands algorithms will probably change over time.