@AmyPhoenix13 Can you borrow a keyboard and mouse from someone to test, just to make sure it is in fact an issue with your specific k+m? Sims 3 is ten years old and may not get along with whatever new driver is being used, but there could also be some other issue with the game.
It would also be helpful to see a dxdiag on this computer. You can attach one directly to a post.
https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/
@AmyPhoenix13 It's worth testing with only one monitor hooked up at a time as well. Yours are two different resolutions and are set at two different refresh levels, either of which could cause in-game lag. The fact that one uses DisplayPort and the other HDMI usually doesn't matter, but if you play on the QHD monitor, test it with a DP connection as well, if possible.
Please also check your framerates while playing. Bring up the cheats console (crtl-shift-C) and enter "fps on" without quotes. A number will appear in the upper right corner, and it should never go above the refresh rate of whichever monitor is displaying the game. ("fps off" makes it go away.) If you have consistently high or wildly fluctuating fps, there are a few tools you can use to cap it manually. Let me know if you need help with this.
My FPS seems to hover around 200 - 400, looking at the number it shows, but fluctuates a lot as can down to less than 10.
@AmyPhoenix13 That's way too high, and not good for your card. But it's usually easy enough to cap fps. The first tool to try is the built-in Nvidia Control Panel, although it doesn't always work (and never works in windowed mode). Setting v-sync to On or Adaptive should cap framerates at the refresh rate of your monitor. If CP doesn't work, you can try Nvidia Inspector (free download). Inspector lets you manually specify the fps cap, which will be 70 if you're using the QHD monitor. 60 would be fine too. Instructions for using both tools are in this post, under the spoilers:
https://forums.thesims.com/en_US/discussion/comment/16540301/#Comment_16540301
