What would cause a 2000 Chevy Cavalier with an automatic transmission to not change gears very quickly?

Just a guess but your problem may be with the transmission filter. I recently changed mine on a 2001 Cavalier (64,900 miles) and found it runs much cooler and better. The transmission filter and gasket cost about $16.00 plus Transmission Fluid (4 1/2 quarts on my 3 speed automatic). If you have a socket set you can do this yourself and save about $50 from what a reasonably priced shop will charge.