"High system impact" is something I will not tolerate, irrespective of how good the software may purportedly be!
Indeed, sometimes they work fine from the start, then they begin slowing things down. Also, if you do not pay attention to what you are doing then you get an infection anyway. People just think well, I have an antivirus I can click anything I want. In 95% of all malware threads in forums, people have an antivirus installed. Yet they get infected, and then the antivirus can not even clean it.
I think having an ad blocker installed is more vital than an antivirus, out of site out of mind.
Here are examples below.
http://imgur.com/LxusfPc
With ad blocker.
Without ad blocker.
The temptation to click on random ads is great, especially if you are morbidly obese and see an ad for miracle diet pills. This concept applies to novice computer users, and idiots who believe more than they should...
Having an effective ad blocker like
Ublock Origin, and software like
Unchecky, is more likely to protect someone than an antivirus. Unchecky will keep the Novice who installs things safe, and it is much much lighter. Add something like
VooDoo Shield to the mix, and the novice computer user will not install a software if there are even two detection's. They will see a detection and be like, to hell with that it's a virus.
Other things like
Anti Adblock in conjunction with
Grease Monkey will help the novice from disabling their ad blocker cause a web site says they need to. An antivirus is a complete waste of system resources, a person could install all of these software that I mentioned and notice Zero slow downs. Also, with emails. You gotta know that you didn't win 1,000,000 dollars and nothing is going to grow your.... well uh, you get the idea.
For all of you who will say, yeah you need an antivirus. I will direct you to re-read these paragraphs that I have just written. Following these methods, many people I have helped are issue free.