Dara could have made a CDF version of the interactive app with sliders, which would have run on your computer and been quite fast and responsive. But the problem is that you would then need to download and install the CDF Player plugin from the Mathematica site into your browser. Some users can't be bothered with this step, which is an aspect of human nature that developers have to deal with.

There is no difference in programming a cloud-served app versus a client-side plugin, as that is just a Mathematica deployment switch.