System specs would be helpful, as well as what game your having issues with.
Each and every game will be slightly different, on different systems.
And it also depends on what your definition of "Best Performance" is.
To some, Best performance is being able to hold 60FPS with x4AA or more, and x16AF, with everything else turned to the maximum settings.
Others, so long as it appears to run smooth and without hiccups, at the games default settings is the "Best Performance", Regardless of the settings of actual FPS.
What I would do to start is set the game to run at your default resolution.
If you have issues running the game, start lowering settings untill it smooths out.
One of the biggest performance hits will come from AA (Anti-Alaising)
Sometimes, there are other tweaks you can do as well, that may help with the overall feel of the game.
For example, Mouse Smoothing and Mouse Acceleration.
Personally, these drive me nuts, and I turm=n them off if the game will allow it to be done. Most often, it will.. although it may be a config. tweak.
While these may not have anything to do with Performance, they do alter the way game responds. To me, it feels smoother interacting with the game world.
Others may disagree and say they cant play without it. Its just one of those things thats a personal Prefrence.
Another is Film Grain and Motion Blur.
These 2 may actually have a small impact on performance.
But even if performance is not a issue, these are best turned off IMHO.
This of course is a personal prefrence again, and not everyone will agree, but to me it makes the games that have them look better if disabled.
I would however stay as far away as possible from Registry Cleaners, and Game Boooster/Performance Boosters.
Reg. Cleaners do more harm than good, and quite simply, offer no performance boost what-so-ever.
If your PC is struggling to run a particular game, regardless what you do, it may be a hardware issue.
If this is the case, you'll just need to upgrade. Theres no software that can take the place of proper hardware.
