Yes, but that doesn't help much against the JavaScriptional (and rendering) bout of hyperactivity that that ensues shortly after the first HTTP response and later on while using the site.
The complexity of embedding all that stuff is enormous. Just block it all for a day and see what that does to the perceived speed of the site.
I have found the biggest culprits are the social widgets and fancy js library. Ads and analytics rarely slows down a site (for me). There are exception though like few days ago I bumper in to a site that had 6 different analytics. That's just crazy, also outside widgets can load their own analytics (I think disqus does that). Things can quickly turn dirty.
The complexity of embedding all that stuff is enormous. Just block it all for a day and see what that does to the perceived speed of the site.