I have not yet used the IP Blocker feature of Moodle, but I'm thinking about it because of attacks indicated in the server's security log. My question is, does this setting have a noticable effect on performance?
Well, it must have some impact on performance, since Moodle is doing some extra work, but I think the impact will be almost zero. You probably could not even measure it.
I don't know a lot about Moodle but would the blocking not restrict on the session level in the first place and prevent performance on the detailed level? That way it should be hardly a problem I think?