Steam is just a platform for Windows & Mac games. On each page of a specific game, there are system requirements which tell you what hardware and software you need in order for the game to run. There are no tweaks for the client so the games will run faster instead you will need to upgrade the components of your computer with more powerful ones (GDDR5 graphics memory, DDR3 Ram memory, Dual/Quad-core CPU, etc.)
Also, updating DirectX, graphic driver and sound driver could help you a lot in case your drivers are outdated.