Achieving top performance for Windows 7 on QEMU/KVM with QCOW2 is absolutely possible and involves a systematic approach. You must move beyond default settings and specifically tune your environment.
Set to none or writeback for the fastest I/O throughput. windows 7 qcow2 top
Download a tool like SDelete from Microsoft Sysinternals inside the VM and run: sdelete64.exe -z c: Use code with caution. Achieving top performance for Windows 7 on QEMU/KVM