I've currently experienced a massive slowdown when using flag "-cpu host" on an AMD Phenom 905e with qemu-kvm 0.11.0 and kernel 220.127.116.11 x86_64. This affects network (tested e1000 and virtio) and i/o performance (virtio). Further there are dozens of log messages from kvm module complaining about:
cpu0/1 unhandled rdmsr: 0xc0010055
Both, the slowdown and the rdmsr-message do not happen when using qemu64 as cpu, but also occur when using -cpu phenom. Thank you for your comments!