time wget -q -O /dev/null http://www.google.com/
time wget -q -O /dev/null https://www.google.com/
There is no obvious difference in time taken to serve those requests.
Obviously there is an overhead, but to me it seems insignificantly small in comparison to the overall request.
I bet 99% of websites out there could convert to SSL, and then pull back the performance lost by just doing a little site optimisation.
-Google's servers are big enough they can easily handle the extra load of encryption without breaking a sweat
-One of the many factors that makes SSL slower is that assets will not be cached in a shared cached (e.g. a proxy server), and will also not be cached in a browser beyond a single session (usually). So, stylesheets, javascripts, and images, must be downloaded to the browser much more often.
Note that your wget test completely ignores this last point, because it is not trying to cache anything for either https or http.
I actually wouldn't be incredibly surprised to learn that shared caches in proxy servers are quite efficient at serving cached content. If you think about it, it's in internet providers' best interest to make their proxy servers good at caching, because it would allow them to provide more content to their customers with less bandwidth, which of course affects their bottom-line.