Looks resolved in beta 307. Signing and exporting to public is now so fast even the first time around that I can't reproduce this condition.
Sep 15 2017
Resolved for me with beta 307. Kleopatra gets launched, starts a few GPG services, then the message gets signed. It takes 20-30 seconds, but that's expected. It's much faster after the first time.
False alarm, this should be closed. It was caused by enabling SMIME support in GPGOL while the sender only had an OpenPGP cert, no SMIME cert. Hence Kleopatra threw a message that it could not unambiguously determine the right cert, then offers only one cert (OpenPGP) for user.