That has definitely not been my experience using many different antiviruses on many different computers. I sometimes see huge differences in performance between different antiviruses.
There can be performance issues due to conflicts or leftovers from a previous antivirus, due its uninstaller not removing it cleanly. However, I find that many antiviruses cause very noticeable drops in performance, from time to time or in some cases always, due to excessive CPU or disk use. I guess on computers with very fast CPUs then there probably won't be much difference from one antivirus to the next. But, for example, most of my computers only have Core 2 Duo CPUs, and on them, I see a very big difference in performance between different products, with some antivirus making my computers run very slowly. My fastest laptop only has a second generation i5 CPU.