It looks to me like they find it really hard to measure performance impact, 12 products received the same score with their in-lab tests (AV-C Score: 88) and are only separated by PC Mark Scores. Their test tells me that the difference in performance impact is really small on a PC similar to the one used in their tests: Intel Core i7 920, 4GB RAM, SATAII HDDs. Since these are not the minimum requirements that many suites have, I think it would be much more relevant to also perform tests on a PC with 1 GHz CPU, 1 GB RAM (32-Bit) or 2 GB RAM (64-Bit) or whatever vendors write on their web-sites as min. requirements.