Now this is only targeted for people who play fullscreen games like oblivion, fallout, world of warcraft, and such. When you have situations in the game where during intense action the game slows down but otherwise it runs fine there are some settings that are very very minor but just buildup when the main action builds up.
One of the first settings that can take up a lot of energy is Shadow Quality. Shadow quality is just as it sounds, it's how realistic the shadows are according to light sources. I don't know about you but I only care about the quality of a shadow when I am not being killed by multiple enemies.
The next that i usually turn down is Foliage Density. Foliage is how much grass, weeds, flowers, etc. shows up when you are playing. Yet again this is something your only going to notice when you character is face down in it after you die due to lag.
Though this next one is somewhat nice to have but if you are still having problems with lag then this is the next to go. Texture quality is how detailed the environment is, so this does take up a lot of processing power to keep this up while many other things are happening. This is the first one that I would say turning down would effect game play experience but sometimes you have to take the hit so you can properly play the game.
Now there are some setting that are more important to some games then others. For example in World of Warcraft particle density is a very important setting due to it being the thing that shows where spells are in effect and tell you where you should or should not be. In Fallout 3 it would be Model LOD which tells the game how far away it should first show the other NPCs or enemies and if you want to be able to snipe well this is something you want way up. Now I can't be listing every important game specific setting, but I suggest you keep an eye out and switch the settings you don't need down, and put these settings up. When it comes to game makers they mainly concern themselves with the people having problems when they have the highest setting on since most problems with other people are just hardware problems, so make sure you adjust the settings to your liking.
Please if you have any request on stuff like what I have been posting leave a comment and I will make a post about it (if I am confident enough in the subject).