> How many billions of NTLM keys can a Geforce crack in a second?

According to https://hashcat.net/oclhashcat-plus/ , it's approximately 2.5 B.

This corresponds to a one time pad message of just under 4 bytes long. The difference, of course, with NTLM is you know when you've found the right value. With OTPs, all decryptions are possible and equally valid.