LVM: is this mirrored? is copy this slow?

if you are creating a lvm mirror volume from zero of some TB, maybe you can use the option --nosync

man lvcreate
 Specifying the optional argument --nosync will cause the creation of the mirror to skip the initial resynchronization.  Any data written afterwards  will  be
          mirrored,  but  the  original  contents  will  not be copied.  This is useful for skipping a potentially long and resource intensive initial sync of an empty
          device.