> Web sites can fix the problem by converting over to HTTPS and marking their session cookies as HTTPS-only, but it's not quite as simple as just flipping a switch. HTTPS will definitely require more server resources to encrypt and decrypt all of its traffic, but there are other potential problem areas as well.
So far, server resources or not, I believe that limited HTTPS support was mostly about little incentive there was for most users to use HTTPS portals. HTTPS Everywhere has been around for some time, but no so many non-tech users were using it.
secure.wikimedia.org for instance is definitely slower than *.wikipedia.org. HTTPS Everywhere users might have noticed that. There are probably various legit reasons for this (hello wikitech-l!), but until now improving response time on HTTPS portals was probably not the top priority of availability/reliability teams.