Turn down all detail in the game (turn it up incrementally later).
Close all running apps, except for the game(s). This includes your email client, web browser, etc. Check your "Login Items" under System Preferences. Ensure you don't have any background tasks that are consuming CPU cycles and/or system memory.
If you have minimal system memory (RAM), it would be a good idea to upgrade since RAM is so affordable now. You can never have enough RAM. I would recommend at least 8GB.
Activity Monitor will also show the CPU load on your system when it's sitting idle. It will show apps that might be running unbeknownst to you.