Which upgrades will increase performance the most?
1. Suspension ( assuming there is rough terrain ) A properly tuned fork will remove road noise allowing the rider to go as fast as possible.
2. Clipless pedals SPD pedals will allow the rider to spin ( by pulling up on the pedals ) in a higher gear than otherwise possible.
3. Hydration system ( for long rides ) Once a rider is thirsty it's too late. They are probably suffering from early effects of dehydration. Camelback type systems keep riders fresh for longer.
4. Wheels lightening rotating weight removes more resistance than non-rotating. Ideally weight should be removed that is further from the center of a rotating axle.
5. Cranks power transfer is huge. Hollow tech cranks have a larger surface area of contact. Hence the power transfer is increased dramatically. Carbon fiber is super stiff, hence transfers more power.