I bought this system from best buy for $280 CAD at first this system had windows 10 installed on it, when i tried to play games on windows 10 they would not work. So I downgraded from windows 10 to windows 7 being able to play games, anyways using the LCD screen it worked fine but connecting it to my Samsung S27F350 (which is 1080p) slowed down the computer hugely i could not even run windows correctly and a reinstall did not help. Even though i connected it to my Samsung 55 inch TV (using HDMI) it was working correctly I do not know if the monitor i am using is bad or not. on the monitor i used VGA and HDMI and for a while they were working with full performance but now using both inputs my Dell Latitude E6420 is super slow.
When you say you're using both VGA and HDMI, are you connecting them simultaneously? If so, then if the TV keeps all of its inputs running as active (rather than just the input that's actually selected on the TV), it's possible that your PC thinks it's connected to dual displays and is therefore doing a bunch of work to render a second display that you're not even seeing. You should only have one type of display cable connected to your display at a time.
If that's not the issue, try updating your GPU drivers if you haven't already. There's no way a display could be "bad" in a way that would cause your system to be significantly slower when using it if the system performs fine when using a different display at the same resolution.
The only display my computer is porting to is my external monitor I also keep my laptop screen shut and sometimes i open my laptop monitor it is just black and i know Intel HD Graphics 3000 only supports 2 monitors. whenever i preform speedtest (speedtest.net) it just fails. This effects everything from chipset drivers to just gaming performance. I am thinking of buying a new desktop with a GTX 1050 to resolve the problem. I will also try to uninstall my PC's builtin monitor from device manager. I also had to reinstall windows many times on my builtin monitor and when i connected my external monitor a fresh install of windows 7 was extremely slow! Write back if you find another solution. (also I updated GPU drivers, didn't do anything)