On a 8 core CCR with quite a few firewall rules, (where fast-path cant be used), we find that if traffic starts to exceed a certain level on some of the routing that is required, other interfaces start to experience jitter and or packetloss.
Its not perceivable to most applications, but it is to one: VOIP. Is there way for the CPU to treat one interface more importantly they another? (Physical ether, as well as possibly, VPLS, MPLS interfaces).
Its worth noting at present the packets them selves are all highest priority TOS/DSCP as well. (Express forward etc)