http://gmailblog.blogspot.com/2008/07/making-security-easier.html
"...We use https to protect your password every time you log into Gmail, but we don't use https once you're in your mail unless you ask for it (by visiting https://mail.google.com rather than http://mail.google.com). Why not? Because the downside is that https can make your mail slower. Your computer has to do extra work to decrypt all that data, and encrypted data doesn't travel across the internet as efficiently as unencrypted data. That's why we leave the choice up to you. ..."