W3 Total Cache doesn't seem to be working with the Spam Free WordPress plugin. With W3 disabled real humans are able to comment, but with W3 enabled I consistently get the following error from the SFW plugin (I've reached out to the SFW developer too)
Spam Free WordPress could not retrieve the password from the server. It may be necessary to do one, or all, of the following. Turn on the Old Password Fields option, turn off Nonce Security, or to turn on Generate Comment Form. Troubleshooting.