I may as well throw my hat into this ring, too. I've been having the issue where closing a tab would freeze my MBP for up to a minute or two (sometimes less). I never noticed if it was only involving sites with media, like YouTube, but having read through pretty much every page of this discussion, I'm certainly keeping my eye out for that.
The last two times I experienced the issue were yesterday and just now, even while writing this post. Both times, I was, indeed, closing a YouTube tab, and my laptop froze, although this time the freeze only lasted a few seconds. I'm using the latest Safari Technology Preview. (I downloaded STP the day it was first released hoping it would fix this issue. Unfortunately, it didn't.) I don't remember anything else about the situation: if the tab was foreground or background, if I stayed in STP while the close was happening or if I tried to cmd-tab away during the closing, etc. All I know is I closed a YouTube tab and my MBP froze for a bit.
I don't recall if the previous 27 pages of posts mentioned this or not, but the freeze does seem to be only visual, in a way. If I keep typing during the freeze, then what I typed will show up after the computer starts working again.
I've had this issue for quite a while, now. I don't recall if I experienced it before El Capitan, but it is very possible, given how long it's been happening to me.
Laptop specs, according to About This Mac:
OS X El Capitan 10.11.4
MacBook Pro (Retina, 15-inch, Mid 2014)
2.5 GHz Intel Core i7
16 GB 1600 MHz DDR3
Intel Iris Pro 1536 MB