We were running heavy loads and there was nothing like a 20-30% hit. I'm not saying you didn't see one but this isn't magic or a black box: we had a few spots where we needed to tune (e.g. configuring our virtual networking to distribute traffic across all of the host NICs) but it performed very similarly to the bare metal benchmarks in equivalent configs.
What precisely was slower, anyway - network, local disk, SAN? Each of those have significant potential confounds for benchmarking.