This happens to me sometimes. Sometimes windows only allows a program to run across fewer cores. You want to make sure that your cpu is fully utilized, and I'll explain how.
If you hop onto Task Manager by pressing Ctrl + Shift + ESC, and where it says "Apps", you want to right click on whatever the game your running is and click "Go to details." Once you do that, the app will be highlighted, right click the highlighted app and select "Set affinity." This opens up a list of all your CPU's cores, if it's not performing well, it'll only show CPU 0 and CPU 1 checked. You wanna click the check box labeled "<All Processors>", then click OK. This makes it so the app you're using will run across your entire CPU, making performance much better.
Hope this helped!
P.S. Task Manager can be a dangerous app if you try to mess around with things. If you try toggling and setting things you don't what they do, I highly suggest you only use and change things you know will work and won't break anything. I highly doubt anything in task manager can break things, but I'm sure there's still a risk of something happening if you mess around enough. I've done the Set affinity setting thousands of times and it's always made games perform better.