(EDIT: Originally posted as reply to another thread, moved here subsequently)
Though it’s probably not related to the same bug or issue which is indeed correctly fixed by “who me” suggestion, I thought I’d mention this since it’s still on the subject of speed/performance within KVM.
I recently went to make a few snapshots of Workstation via virt-manager, only to discover the button for doing that didn’t exist. Only when I looked into it did I realize that the version of Virt-Manager available in my distro’s repo (Linux Mint) was quite old at version 0.95. Virtual Machine Manager is at 1.21 on their website and has a lot of new added functionality. It was quite an exercise trying to find and add a repo which contained the newer version, but I finally found one at Getdeb.
After installing the new version, not only did snapshots become available through the graphic interface (I know they can be done through virsh command line) but the overall speed and performance improved noticeably too.
So, if you are using Virt-manager with KVM, it may be worthwhile to check the version currently installed. I know it’s mostly a graphical front-end for qemu-kvm/virsh, but for me it definitely made a difference in speed within the VM’s. I don’t know which distros already have the up-to-date version, but Mint/Ubuntu apparently don’t seem to.