[jayflux]

"Whenever you attach an event handler on scroll, it can impact page performance because the javascript needs to be evaluated before the scroll event can continue AFAIK...going from some old Paul Irish talk on performance."

This isn't true no. Scrolling is an asynchronous event, in this case meaning it will happen regardless (without waiting on any JS event handlers), the JS event triggers after its happened. This is the reason you can't prevent scrolling from javascript.

Unlike clicking links, or submitting a form, the browser doesn't wait for any JS event handlers to return before doing this action. This was conscious decision to stop scrolling becoming too heavy or slow, or worse...crashing the page.

[emn13]

It's not true in some cases, sometimes. It's still quite easy to cause performance problems listening to scroll events, because even if scroll is partially asynchronous, the webpage itself is not - the browser will wait to render new content if the page is blocked.