I actually tested and while on full system scan, did not see any slow downs, cpu only went a bit higher than normal and memory did not really go above the 70/80's benchmark, although their website do claim 100mb.
I have fired up VMware (since I am still having trouble with VB), did some malware testing and although some bypassed, most were caught so detection rate ain't that bad. I then compared the same links and malware files with NAV, KAV and Avast. Sure NAV and KAV had better detection rate and Avast didn't come that far from Eset, but I have to say that KAV and NAV while had low footprint, when the full scan started I saw the memory usage increase quite a bit.