I know someone who was running Norton, about a year ago, on a brand new laptop, and it was slowing down her system pretty good. Not to the point of things being substantially slow, but it was definitely noticeable to me. I don't think she really noticed, because there was no complaints about speed (then again, her last computer was terrible), but Norton took its toll. You really can't prevent a security software from slowing a system down; it's just a side effect of running that kind of software.
But, that aside, I wasn't really talking about speed. I was really talking about overall user reviews. I just haven't heard too many people who are genuinely happy with the job that Norton does. I've definitely not heard a positive review from anyone I've talked to in person; I've only seen positive reviews of Norton on the internet. I should definitely consult my local computer shop, though... I'll have to make a note of that.
On a side note: I will say that I've had one noticeable problem from every security software I've run... video streaming. For some reason, every single security software I run has to screw up my video streaming experience. It doesn't matter what website I'm using, I have that problem. It's weird. The only time I have a good streaming experience is when I'm not running AV software. I think part of the problem is the overwhelming amount of flash video on the internet, as Flash itself doesn't seem to like letting me multi-task while I'm running AV software. It could also be my browsers causing part of that problem, but I'm not about to leave them for anything else.
And yes, I'm having this same recurring problem with Bitdefender. It's relatively lightweight, from my experience, but it still bogs down slower machines in some ways.