The smoothness is what they’re trying to make happen, here. If you use this app – in a Chrome web browser – you should find that it works without a hitch – and that it works without any stuttering or, as they call it, “jank”.
This web app is meant to “launch almost instantly, and then [perform] a smooth UI even when it’s doing heavy work, including using Web Assembly to do more with codecs the browser doesn’t have baked in.” You can access this app over at Squoosh dot app. It’ll work in a Chrome web browser on a desktop machine, mobile Chrome app, or in Chrome OS. And it’s basically magical.