Back from a little hiatus!
IMO any program that takes up system resources will have an impact on the system. The real question however, is whether said impact is noticeable? The problem with Windows is the infinite number of hardware/software combinations available, which makes it difficult to get an accurate assessment. Looking at the responses here and in other threads, you will notice that all security programs will have varied results when it comes to performance. Some say they find Kaspersky light, others says it's heavy, same goes for Norton, WD, BD, etc.... Even tests from the likes of AV-Comparatives aren't overly accurate either. Again its just a quick snapshot in time and doesn't take into account what the overall impact will be long term.
All in all I think they best solution for anyone is to try the program(s) and see for yourself. After all it doesn't matter what any of us, or AV-comparatvies, etc... say, all that matters is whether YOU notice the impact. As I said above, everyone will have different hardware/software configurations and everyone uses their computers differently, so take advantage of the trials and go from there, it really is the only way to know for sure.