hello,
CEF rarely needs to drop a packet, even if it is the first packet of a new flow, it is able to operate much more efficiently than Fast Switching. And because it caches the entire routing table, it is even able to do packet-by-packet round-robin load sharing between equal cost paths. CEF shows its greatest advantage over Fast Switching in situations when there are many flows, each relatively short in duration. Another key advantage is that CEF has native support for QoS, while Fast Switching does not
check this link for QOS
please rate if this helps
thanks