Interaction to Next Paint (INP) and First Input Delay (FID) are metrics used to measure how responsive a website feels to users. FID tracks the time it takes for a website to respond when you first interact with it, like clicking a button or tapping a link. INP, on the other hand, looks at responsiveness throughout your visit by measuring the time between your interactions (like clicking or scrolling) and when the page actually updates or reacts. Basically, FID focuses on the first impression, while INP gives a bigger picture of how smooth and responsive the site is overall.