A simpler alternative: It is becoming an increasingly accepted alternative to use TLS all the time, rather than switching back and forth between secure and unsecure connections. The bulk of additional processing time is spent setting up the secure tunnel, but this is only done once and cached (typically). The symmetric encryption of subsequent traffic is very, very fast on modern processors. It’s somewhat out-of-date thinking to believe that this would cause a server overhead or scalability issue.
In a recent blog post, a Google engineer reported that when they switched to HTTPS-only for GMail, they found their server overheard increased by only 4%. (Can’t find the citation.)