How does expensive cranksets affect the performance?
Firstly, most people use higher-end craksets, because they enable higher number of speeds in the rear by using a narrower chain. A 7 speed crankset cannot be used, if you have 10 or maybe even 12 (13!) speed chain.
That affects the speed you are going only indirectly. What also comes with higher-end cranksets is smaller weight and stiffer construction. For many people the weight difference is not that big and not worth the money. One can lower the weight more cheaply elsewhere. The stifness is a marginal point to most.
Many carbon fiber frames also require press-fit bottom brackets. That means you cannot use the cheapest square taper bottom brackets that are threaded and therefore the (often) cheap square taper cranksets. The frame of your bike then requires you to go somewhat higher (although less expensive options still exist).