[libvirt-users] How can a bridge be optimized?

Hi, Netperf tells me that a KVM VM using a bridged connection to a system across the Gigabit LAN sees on average about 1/10th of the bandwidth the host sees. This is on very lightly loaded hosts, although with multiple low-use VMs. I've tried adjusting the CPU throttling after seeing that mentioned as a possible factor in slowing down a bridge, but it has not much if any effect here. Is there a good guide somewhere to what might make a difference with this? I recall the kernel recently adding a new alternative for handling VM interfaces, but can't recall what it was called. Would that be a marked improvement? Is it supported by libvirt? Thanks, Whit