It's probably because it's constantly checking if the target inventory has space. Hoppers are slower and so use less CPU because they are checking less frequently, although they are checking whether there is space for any of the four items that they hold whilst the relay is checking only for the one kind of item that it has sucked into its invisible holding slot. The insertion speed is one of the advantages of relays, a hopper can't keep up with a Recycler with 12 Overclockers in it, whereas a Relay can. That performance comes at a price, so I use hoppers unless I need a) speed or b) insertion from somewhere other than the top.