Under normal configuration, it is the RADIUS server that is keying/rekeying for the clients authenticating. Depending upong your RADIUS server capabilities, it can be a resource hungry process.
AAA Fast connect (or simply EAP-termination), offloads this function from the RADIUS server. The keying/rekeying function will be done on contraller hardware (atleast Aruba does it in hardware) now and RADIUS will only be authenticating the clients only.
That essentially means by enabling AAA fast connect, you are freeing up some resources for your RADIUS server. But with modern deployments, I am not sure if that will add any signifact performence effect. I never use this option.