The most common reason for poor video performance is that the customer has simply plugged in their monitor to the wrong port in the back of the system. Our systems have both Onboard and Dedicated Video (Video Cards).
When you plug in your monitor to the onboard video ports near the USB ports in the back of the system, you are not taking advantage of the power & speed of the actual video card you purchased.
You are running onboard video that utilizes the CPU directly.
What you want to be using is the video card, it is located further down toward the middle of the back of the system, you should see an array of ports similar to this, but maybe reversed since this image shows the card backwards to how it would be installed in your system. Just imagine this image rotated 180deg:
Please note, your video card MAY have a slightly different layout, but it will look very similar.
You can use any of these ports, but HDMI is not recommended, especially if trying to run more then one screen. For single screen, you should be OK though.