I've had a question about PFS for a while now: How 'expensive' is it to implement? First up, let me state for the record that I don't have personal experience

I've had a question about PFS for a while now: How 'expensive' is it to implement? First up, let me state for the record that I don't have personal experience with running SSL on a server. But back in the day, generating a public/private key pair for PGP was a semi-expensive operation. Definitely not the kind of thing I could imagine running on a per session basis. Since PFS requires per-session public/private key pairs, how "costly" is it? (I understand that the goog uses ECDHE, so I guess I'm asking for a general feel for how quickly and efficient this compares to RSA key generation.)