Description of problem:
Libvirt uses a very inefficient way to probe qemu for CPU feature flags, which is done by using ~400 QMP calls and responses to fetch all the required information as each flag is queried one-by-one. This is due to the lack of a better interface in qemu.

This takes a significant amount of time during the startup of the VM:

2022-02-07 11:41:50.811+0000: 1758242: info : qemuMonitorIOWrite:402 : QEMU_MONITOR_IO_WRITE: mon=0x7fff9c1d2a20 buf={"execute":"qom-get","arguments":

,"id":"libvirt-8"}
len=115 ret=115 errno=0

[...]

2022-02-07 11:41:50.950+0000: 1758242: info : qemuMonitorIOWrite:402 : QEMU_MONITOR_IO_WRITE: mon=0x7fff9c1d2a20 buf={"execute":"qom-get","arguments":

,"id":"libvirt-395"}
len=129 ret=129 errno=0

In my example case it's 0.14s just to probe the features.

Libvirt should work with qemu to provide a better interface where we can fetch multiple flags at once.