For people in need of a frame rate boost (more frames per second, or "FPS"), iRacing has recommended setting the "Crowd Detail" on "Low" in the in-game graphics options -- "High" detail just adds backsides to the people in the crowds, which is the part you usually can't see when driving anyway. 

In the "Replay" graphic options you'll want to put them on high though, because the TV camera views often pan behind the people in the crowd and this could be noticeable in replays.

This is an easy way to gain a little performance increase without sacrificing any real visual goodness while driving. This kind of thing becomes even more important when you're using low-end or mid-range hardware to begin with, because when you're racing online, framerates can be critical, so EVERY little bit helps.