I hope by smooth you're referring to the graphics being vector vs bitmap, because Flash performed really badly once you added any kind of complexity to a scene. (edit: simple things worked brilliantly though, and often that was enough.) The key innovation of Flash was and still is the creation side of things in Flash Pro rather than the runtime.

My main experience was with Line Rider. I didn't create it, but I became involved in the community and started making tweaked versions with tools that made it easier to create tracks (as opposed to versions that let you do things like change the physics or the rider).

Every line you added to a track caused a new MovieClip to be added to the scene that had a line drawn in it. By the time you had around 3,000 lines or so you'd start to get reduced framerates. 3,000 lines sounds like a lot, but is tiny when you consider tracks that have accompanying artwork.

A few years ago, we released a JS + WebGL rewrite of Line Rider that has a physics engine that's bug-for-bug compatible with the original (you can play it at https://www.linerider.com/ if you're interested - sorry if that's too self promotional). With access to WebGL and a massively faster ECMAScript runtime, the JS version runs multiple orders of magnitude faster than the original Flash version.

It's now possible to work with tracks that have hundreds of thousands of lines without experiencing frame rate drops as opposed to just a few thousand lines. So as for web platform tech underperforming from a runtime point of view, I disagree.

The problem is that all the ingredients are there to make something great, but the developer effort required is monumental in comparison to how fast it was to make something work with Flash Pro.