Technically, yes, HTTPS is slower than standard HTTP. But the difference might as well be negligible. TechCrunch makes it sound like Facebook have gone to the end of the Earth to do something good.
The article gives the impression that Facebook sacrificed everything in the name of security. That’s just not true. It required a menial level of effort (relative to Facebook’s overall talent pool) and a few more servers. Facebook did it because it is easy and is probably something they should have already done years ago; it’s as simple as that.
A lot of blogs, especially TechCrunch, need to understand that company and product changes aren’t required to have a secret agenda behind them.