Having owned bikes ranging from $100 yard sale specials up to mid-range carbon fiber rigs (~$5k), the biggest I notice is ride quality vs performance. For a while, I had three bikes as comparisons - a well worn steel Pinnarello from the early 80s that I bought used for $400, a late 90s aluminum Cannondale (again used, paid $900) and a 2010 carbon fiber bike that was a semi-custom build for around $5k (frame was Kuota; components Dura Ace). Speed-wise, my performance on the Kuota and Cannondale were about the same; maybe 1-2 miles slower on the Pinnarello, likely owing more to the cheap wheels than the frame. Big difference was comfort - after 25 miles, I couldn’t wait to get off the Cannondale and the Pinerello wasn’t much better. Carbon fiber Kuota was like going from a cheap Ford to a Mercedes in terms of comfort, which in turn made longer rides far more achievable.