When it comes to slowdowns, one thing worth considering is the choice of software. Anti-Malware program A won't have the same performance impact as program B. So, while MS Security Essentials might not slow your computer too much, another program probably will.
Also, one should probably take the speed of the CPU into consideration. The number of required to do a job cycles could be multiplied ten-fold, thanks to anti-malware software, but if you have 16+ ridiculously-fast cores, you probably won't notice any difference. IMHO, if you really want to get a sense of performance, you should test on an Intel Atom processor. The speed of any one product will immediately become apparent.
Personally, I feel like software development in general has gone downhill during the 2000s. As of 2006, it felt to me like any commercial software was too bloated to run on my 2002 computer. Even something as simple as a CD burning program would completely hog my CPU (to this day, I still refuse to use Nero software). Now, I know my computer wasn't that slow, since IMGBurn and CDBurnerXP would only use 2-5 percent of my CPU.
This trend has only gotten worse. Now, you need many gigabytes of RAM just to get your system to boot in a reasonable amount of time. Running more than one or two apps on a machine with only 1 gig of RAM often requires either carefully choosing which app to use for the job (that is, figuring out which app has the least amount of code bloat), or mentally preparing yourself for seemingly-endless thrashing.