There will be some basic low level calls, just like the "*NIX Server Load Limit" does, but it is minimal. There will always be some level of SQL calls because the script needs to determine several things:
- Is this a logged in user?
- What is the setting for the unregistered users threshold?
- What phrase is needed to relay to the user that they are denied?
- Load the hooks so this plugin will work.
- etc..
There really is no way around that..