SSL was implemented long time ago for mining already but was removed again due to the increased network lag.
Lags can only create a pool if it spends a lot of time to encrypt/decrypt data, am I correct?
SSL connection is based on same sockets and it takes almost zero time on client side to encrypt/decrypt 1Kb of data.
I'm curious about server load, is it too expensive to use encrypted connections for you?
Google is a fan of SSL on every site, if it needs it or not, and they care vary much about speed of the internet. If there was a speed issue, it may be different. SSL connection adds a tiny delay to the initial connection, and after that it's negligible.