I'd phrase that as "local" and "synchronous".

"Achieving" latency using local JS is not hard, just do some heavy compute and/or do the same via the browser's layout engine.

Try and do some real work in JS, for example, generate Sudokus, render raster images from DOM fragments, whatever: you'll see latency.

If you want non-blocking latency (like from a network request), use asnyc APIs and a Worker or other natively non-blocking API. Or simply a Promise wrapping a setTimeout.

But latency in synchronous and local JS is an everyday problem. So I think your phrasing is wrong.