BlackFog has been specifically designed to have little to no impact on your machine in terms of performance. If anything you should notice your machine is noticeably faster because it is preventing the transfer of vast amounts of information from your machine over the network.
From a CPU perspective BlackFog uses around 1-5% (depending on Operating System and processor) during normal network operation and 0% on idle. Your browser typically uses 20% or more depending upon the site you are visiting. BlackFog also uses only small amounts of other system resources such as memory and drive space.