The file you specify as a key file should be at least 512 bytes long and consist of near-random (in the sense of incompressible) data. The "+makerandom=length filename" option of PGP is an excellent way to generate such a file. To securely deliver a copy of the key file to the person you wish to talk to, encrypt it with their public key using PGP and send it to them, in ASCII armoured form, via electronic mail.
Key file encryption is by far the least demanding on your computer; it requires almost no additional computation. The level of security provided, however, is much less than the other encryption options, and should be viewed as a last resort alternative to transmitting in the clear if your machine is too slow to use any other form of encryption.
Speak Freely will continue to correctly receive unencrypted sound from a given host even if an key file is specified for the connection.