Copr spin up a new VM for every task. Right now we have more that 20 VM running in parallel. That is approximately 4-5 VM per one hypervisor. And building package is mostly disk intensive task. Those days when you have to compile everything are gone. Usually it is installing build requirements, unpacking tar file, moving packages to build roots, creating rpm package itself... I discovered that Mock can easily generate disk bandwidth more than 20 MB/s. Times five VM and you have 100 MB/s which too much for the RAID disks in a Compute node.