Malwarebytes (the free version) and Norton Eraser are both On Demand scanners. This means they do absolutely nothing until you manually open the program and start a scan. So no, they won't slow anything down unless you are currently running a scan.
They are both perfectly fine to run alongside any antivirus program. In fact Malwarebytes is highly recommended as a means of supplementing your protection. I recommend most people to run a Malwarebytes scan once per week just to check for anything their main Antivirus may have missed.
Slow downs and conflict issues start to occur when you have more than one antivirus program using Real-time protection (sometimes referred to as on-access scanning). Someone above mentioned using...