The source of the tests is the AV-TEST Institute. I don’t know if that’s good or bad but it’s a start.
If you click on the individual product & look near the bottom of the page it states the average slow-down of the computer.
Industry average is 10 seconds.
Bitdefender is 10
NIS is 11
MS Security Essentials is 9
I looked at a few others and Webroot was 0 and Trend Micro was 17.
NIS is 1 sec slower than Industry average & MS Security Essentials is 1 sec faster. That does not seem like a big difference considering some products are 14 & 17 secs.
I know several years ago Norton supposedly slowed down the PC. But over the last few years I have not seen that mentioned in any review. IOW they have improved in that area. Again I’m just going by reviews that I have read. I’ve used Norton & Avast & am considering trying Bitdefender. So I’m not hung up on any one product. But if I’m going to use protection I feel it should be one of the better rated products.