1 Turning it on will give you a second opinion, which is good.
2 It is not active protection, so there is no problem running it with another AV, as many posters already pointed out.
3 When it is scanning, it might slow down your computer. But if you leave the lid open on your laptop when you walk away from your computer for a period of time, it will do the scan while you are away. Same principle with a desktop. Leave it running, and signed in to your user account. Okay for it to be locked (showing lock screen)