Is this really true ? I would love to experiment this if I can have longer laptop battery life by disabling JS.

CSS overrides to get rid of box shadows and rounded corners can also make a significant difference in scrolling speed. Essentially every browser out now lets you define a user css file, just include something like:

  * { box-shadow: none !important;
      border-radius: 0 !important; }

etc. YMMV, and you may have to include the corner / browser specific variants, I forget the details. Box shadows with a large spread still perform terrifyingly poorly in browsers, though small ones tend to be fast enough to not notice on modern machines.

Bookmarklets can do such things on a case by case basis, for any adjustments you don't want to commit universally (via user.css).

It's true, and you can.