Cessna 172 Skyhawk (G1000) Slow at cruise

I’m not a real pilot so I’ve only been exposed to these concepts, but wonder how much impact they really have? My overall understanding is if you’re trying to save on fuel and parts wear-and-tear then you always LEAN. If you’re trying to get to your destination as fast as possible then just floor it! Is that an oversimplification of that concept?