ScrollTrigger does NOT constantly watch every element and check its positioning in the viewport on each tick. We're obsessed with performance and that'd be far too costly. Instead, ScrollTrigger does the processing up-front to figure out where the start/end points are in the natural document flow. In other words, "this ScrollTrigger will be active when the scrollbar is between ___ and ____". Then, it debounces the "scroll" events and only updates things on the next requestAnimationFrame, perfectly synced with GSAP and screen refreshes. It ONLY watches the scroll position. Period. That means it's FAST.

ScrollTrigger does not use IntersectionObserver under the hood because it lacks the necessary functionality and compatibility. You can certainly use IntersectionObserver and ScrollTrigger together.