Chainrings, cassette, and wheel size impact the the final drive ratio. For every rider, there is a comfortable cadence range (most fall into the 70-100 rpm range on flat ground). For different gearing combinations, that rpm range will produce a speed range. The strength of the rider determines how fast he or she goes. Certainly the chainring can be a limter on a steep climb or a long descent if it's too big or too small.