Double-check that you have Nvidia Prime configured/selected. It's been a while since I've used Mint, but... try this...

1. Open the App menu (bottom left, same as in Windows)
2. Type "nvidia' and this should show you the NVIDIA X Server Settings app. Click on it to launch.
3. Double-check that you see all the drive info, including the driver version. Close out the app of all looks "right".
4. In the Mint system tray (lower right), click on the Nvidia icon (if you don't see it, open the app menu, type "startup" and make sure "Support for NVIDIA Prime" is enabled) 5. Set the profile to "Active profile" (it's hard on battery life in this mode). This forces everything to run on the Nvidia card only...
5. Test your games. Do they work better? If yes, you've found the root cause of the performance issue... if not... Hmmm, I'm not sure, then it's time to try other things.

My experience with this is that Nvidia Prime was not being enabled/selected when I was trying to game. If this (forcing everything to launch on Prime) works and your games are working at a more acceptable performance level, you can leave it in "Active profile" at the expense of battery life... or you can set up the On-Demand profile.... or explicitly switch between Intel and Nvidia, using Intel for all non-gaming things and pop it into Nvidia when you want to game.. lots of possibilities depending on how you want to use the computer. :-)

BTW, an alternative to the systray method is simply setting the profiles right within the NVIDIA X Server Settings app (the last menu item on the left nav menu within the NVIDIA app). I just find that the systray icon is a quick/easy way, and it's worth knowing about.