It can help, but still has to be applied carefully. A fast initial load is pointless if a bunch of widgets are making the page jerk and jump all over the place for the next 30 seconds.

The strategy I've used is to serve up a light frame with the meat of the content, while keeping all ajax/external site dependent stuff hidden (with equivalently sized placeholders to avoid shifting layout and scroll), then doing a single subtle fadein of the additional content when everything's ready.

You can tweak the timing--like if you have a few bits of ajax content from your server that take 1-2s to load, and a few social buttons that are taking more like 5-6s, you'd probably want to fade those in separately in two steps. The point is to carefully orchestrate how elements appear to the user, not just bombard them with everything at once.