Well, most noticeable in those two, but it occurs when sites are loading into the browser. I mention it with fb & youtube because as you say, they are data heavy pages, so I can see the effect as more pronounced.
A you tube page may load for example for up to a minute as the streaming video data is transferred and while that is going on, the mouse becomes fairly unresponsive.
Didn't used to do it, now it does??