Should I buy a new Monitor?

Hi everyone, I usually play the sims 4 on my mac but with the amount of cc and mods that I have my game lags a lot. I started using a desktop that I had at home but my game is even more laggy now. I have no idea if the issue is the big box (I think that’s the PC??) or if it’s the monitor. The big box(the PC I think) is a Dell Inspirion and I’m pretty sure the guy who sold it to us had upgraded some part but I’m not sure what, it has 2T of space but i don’t know what other things I should be looking at to figure out of this is what’s making my game lags. The monitor that they sold to us is an Asus IPS HDMI with resolution 1920x1080 again I don’t know what to look for in this kind of things 

I’m sorry if this was hard to understand or if I didn’t give enough information on both the PC or the Monitor I just know that it takes 20 to 25 minutes from clicking open the sims to when the screen actually turns black to open the game and then it’s another 25-30 before it even starts doing anything. 

If anyone has any recommendations on what I should upgrade or what I should change I would gladly appreciate it thank you ☺️