I had to install each one and try them (for Win 8.1 64x; I also looked for AVs that did not have popup ads or that could be disabled). This happened after BF slowed down my system a few weeks ago. I clean uninstalled and reinstalled, and the results were the same. I then tried KSC and there was some slow down in file browsing. After that, Panda Dome, and it felt the same as KSC until a few weeks ago.
Right now, I'm using Avast. It's OK, so far.