It does impact system performance, the same way everything which behaves like HitmanPro.Alert will impact on system performance. The 0% CPU doesn't mean it doesn't use any CPU usage, every running program uses the CPU because it'll be running in the background, I don't know how Task Manager works with it myself...
HitmanPro.Alert works with process injection which means the memory of other running programs will increase; the memory usage of the monitored programs won't use a lot of additional memory since HMP.A is genuinely light but it depends.
Since it injects into processes, there must be a reason to need to do this... Hooks! Hooks will allow them to log the behavior of the running program. This means when the hooked (or "detoured" is a more professional term for it) functions are triggered (so when the targeted functions are called and it leads to the custom code written by HMP.A alert before Windows processes the API call properly), they log this information which will be beneficial in identifying certain behavior.