I firmly believe the crawler won't download the plusone library for a few reasons.

- It may not run javascript at all.

- It doesn't need to be checked for malicious content.

- If all else fails, it might have it cached already.

Even if external assets are computed into a site's speed, the overhead is negligible when it comes to search ranking.

Google Instant's previews show the +1 button, so I see no reason why Googlebot wouldn't too. I guess Google wait until window.onload fires instead of just timing the server sending the 1 HTML file. Google could also use data from Google Toolbar users but I doubt they do.

As the article says, even with a primed cache, the +1 code slows down page load. It also said the overhead can be 2 seconds. If you're aiming for fast then that could double your page load time. External assets are very likely to be calculated into a sites speed as they generally take up quite a bit of the page load time.

Of course, no one knows how much of an effect page speed plays in Google's algorithms. My bet, however, is that adding +1 to a fast site could easily make it drop a few places in the SERPs. If your site is already quite slow then an extra 2 seconds may only be a few % slower.

You're assuming search ranking is primarily based on speed. It might weight the ranking, but relevancy must be still dominant.

I mean, come on. Sites with maps, ads, or even big images can take up more than two seconds. It's not a big deal. Your analytics tracking code slows everything even more.

And it's the same network, the crawler might just go to the next server room.

The page speed that Google uses in it's ranking is (in part?) collected from the Google toolbar installed on local clients. The JS is only cached for 6 minutes. So it seems this will indeed negatively impact your search rankings.