@Rich, I found the answer myself, by try and error:
from tc I see: /dev/vda: LABEL="QCOW2" UUID="69b2bd7f-5108-43b9-94d8-da4800b53851" BLOCK_SIZE="1024" TYPE="ext2"
case 1: waitusb=10:LABEL=QCOW2 => countdown stop at 7, so it starts in 3 seconds
case 2: waitusb=10:UUID=69b2bd7f-5108-43b9-94d8-da4800b53851 => countdown stop at 7, so it starts in 3 seconds
if all upercase UUID (as I see in 7zip) it still successfully finished, but:
case3 : waitusb=10:UUID=69B2BD7F-5108-43B9-94D8-DA4800B53851, countdown stop at zero.
so blkid UUID is case sensitive, prone to error if manually input; so LABEL= will be.
So it is the same speed 3 seconds (before staring to load tcz), as with simple waitusb=3; Maybe qemu has a bug.