I increased my speed by nearly 20% on one of my bikes, just by changing tyres. I improved imes by 5% on another just by going clipless, but my commuting time over 10 miles was barely different between a road bike, a cyclo-x and a 20 year old MTB.
Speed depends on surface, gradient, junctions, traffic, fitness, destination & incentive, wind... and a whole lot more. However, (up to a point) an expensive bike with better kit works, feels, and looks (let's not deny vanity) so much better that some people think it's worth it, even if the engine is still too old, too heavy, smokes too heavily and drinks too much "fuel".