Like any other vehicle...tires/wheels make a big difference.

I went from heavy, cheaper 19mm wheels to 25% lighter 23mm wheels. The brand is irrelevant (although I'm sure if you look it up...you can trace my posts). Better hubs, better spokes and a lighter/wider wheel made an immediate impact on ride quality. Not only was the ride smoother, but the handling and braking improved as did top speed. I did some "runs" where I can coast down a large hill, mostly out of the wind (trees on both sides). A simple "coast test" showed me an immediate pickup of a couple mph (I've made this run many times and have a good amount of data). It was immediately noticeable how much faster my bike picked up speed when coasting.

Now...I don't go around coasting all over the place but I wanted a simple test. It is noticeable how better they ride. Like a good saddle, or comfortable shoes or nice kit...you can tell.

My wheels set me back $625. Not cheap but no king's ransom. Had I dropped $2500 and got the same results, I may feel screwed. For my $$...it was a solid upgrade.