How do I optimise GPU and CPU usage?

I’m building an open world zombie game and I need to know how to optimise it to run well on lower end PCs.
I’m using tillable textures (TGA, 128x128) for things I want to have detail on and a colour palette for the things I don’t mind being mostly plain. The Models are low poly and modular but this is as much as I know on keeping it easy to run on lower end PCs.

Thank you all in advance.