Laptops generally take longer because the disk generally is slower, 5,400 RPM vs. 7,200 RPM.
You have to look at the same 'year' A/V or Suite from different vendors. Each year they all improve usability generally. Almost any when compared to how they operated 2 or 3 years ago will be better. In addition, if you've upgraded your computer, even the older ones would work better.
I've got McAfee and my wife has Norton, even with out computers having different spec's, I wouldn't call one better than the other. I can't even say which is less intrusive? Most have settings you can customize to avoid messages and warnings.
Free suites, I don't know? The few I've used seemed OK? However, security suites are relatively cheap. I got a 3 year deal with my computer for $40, and my wife's Norton was free after rebates. Somehow I just feel (probably wrong) that one that you pay for have better support (not really) and react faster to threats. Just my feeling, no hard data.
The real test is how much system resources does it use AND does it 'slow' you down? High capacity fast system, all probably would pass this test. Slow older system, some might work better than others. Second test is does it PROTECT you against new threats... and there is no way ahead of time to know this. Bottom line I guess is that if you have a dual core of better processor, fast hard drives, and sufficient RAM, almost any will do, free or fee.