I can't find anything specific at the moment but anecdotally I remember seeing this and being told it hurt performance to encrypt everything. The "solution" was to only encrypt sensitive pages like forms for credit cards.

I'm sure there was some substance to it at the time when computers, networks and browsers were slower but I also completely ignored that advice at the time and always used SSL everywhere on sites I set up.

I've never manged a very high traffic site so any extra overhead from SSL was negligible for us.