>>HPC Challenge wouldn't build under QEMU-KVM since the reported CPU string was unrecognized as "QEMU Virtual 1.7.0"
Just use -cpu host as cpu model. If not, you don't use most cpu features (sse2,ss3,...). It should give you a big difference if benchmark use theses cpu features.
(It could be great to have the full qemu command detail, to see which options are used for cpu, disks,....)