I'm trying Panda Internet Security at the moment, and so far it has been extremely light. I have only been using it for a few hours so far, which is not long enough to get a accurate idea of system impact, but does give some indication. When I open one of my downloads folders, with hundreds of installers in it, with a lot of antiviruses, it can take quite some time for Windows Explorer to finish reading the folder. But with Panda, this is not the case, even the first time I open a folder.
So far in terms of system impact so far, I would rate it close to Avast, AVG, ESET, K7 and Norton. All of which received higher performance scores. Also it is much lighter (for me anyway) than Emsisoft, Quick Heal, Trend, and Windows Defender, which also were rated higher. In partiular Emsisoft and Quick Heal caused a very big drop in performance.
I know that the system impact of an antivirus can very greatly from one computer to the next. But usually if an antivirus is going to cause slowdowns, it will do so on my main laptop which has hundreds of programs installed on it, and lots of processes running.