OS X 10.11(.1) Randomly freezes while browsing
I kept having this too. After using Youtube for a while, Safari froze and was then on behaving very sluggishly even if restarted. Only reboot of the whole system restored Browser back to normal (until Youtube would freeze it again).

The problem seems to have disappeared along with the latest OS X 10.11.2, so I recommend you just update the OS X.