I used to have a very outdated laptop with just enough RAM, and a sub-par CPU and GPU.
Startup: Open Run and type
msconfig. This will open the Windows utility which contains all the services and programs that are set to start when Windows launches. (In Windows 10, the startup is located within the task manager) Go trough the programs and disable anything you don't use all the time. I'm talking any bloatware that you may be installed by your manufacturer. Software that is not needed when gaming like torrents, Teamviewer, or anything like that. Even Skype and Discord, and you can use the mobile versions through your phone.
Make sure you close any software you have open, that is not related to the game. If you're playing a standalone game, close BattleNet and Steam. If you're playing Overwatch, still close Steam. Definitely make sure you not only close you browser, but actually go into the task manager and make sure you kill any instance of the browser. Often, when you close Chrome, all the tabs can still be seen in the task manager. (In Windows 10, right click on the programs you want to close -> go to details -> right click -> end process tree. Always "end process tree" and not just "end process". This tries to close the entire program altogether and not just say a tab in Chrome.
Open your machine and clean it. Regardless if it's a laptop or desktop either learn how to do it yourself or take it to someone that will take it apart and clean it like new. Just using the vacuum or an air can won't do much good. I usually take everything apart even the fans and clean between the blades. I clean the cooler grills, and recently I started replacing the paste as well. Thermal Throttling can really be an issue quite frequently.
The above steps can sometimes translate into several dozens of frames per second gained.
Only doing the above meant I could play GTA5 at all.
I've also tried Overclocking, but given it was a laptop I didn't get anywhere.