"Proofpoint’s researchers have observed that most websites were slow to enforce the use of HTTPS because the encryption it uses to secure the connection slows down the web experience..."
Are they saying their researchers regard using HTTPS as significantly or just that the webmasters' perception is that it will slow their site down so they don't so it.
Overall, I would expect that https is unlikely to have any noticeable different to any modern webserver, unless Proofpoint know different (and this is now showing as a sub headline for this story "Er, what about latency, bro?")