tl;dr: Use getEventListeners in Chromium/Safari DevTools to get an idea of what’s being run “concurrently” and vying for resources. Note that getEventListeners only exists in the context of DevTools.

getEventListeners(window)

{ error: Array, beforeunload: Array, load: Array, resize: Array, focusin: Array, ... }

getEventListeners(document)

{ keydown: Array, mousedown: Array, visibilitychange: Array, scroll: Array, click: Array, ... }

For example, you can use getEventListeners(window).scroll to investigate how many scroll event handlers are listening and running upon scroll and what the bottleneck could be for a not so smooth scrolling experience.