bootc install --composefs-backend takes much more time than the default ostree backend

Run the following bootc install to-disk command, (the container image is already pre-pulled to the host)
```
podman run -ti --rm --privileged --pid=host --security-opt label=type:unconfined_t \
-v /var/lib/containers:/var/lib/containers -v /dev:/dev -v /sys:/sys:ro \
quay.io/centos-bootc/centos-bootc:stream10 \
bootc install to-disk \
--generic-image \
--composefs-backend \
"$DEV"
```
with `--composefs-backend`, it took
real 1m47.659s
user 0m0.133s
sys 0m0.331s

without `--composefs-backend`, it took
real 8m41.792s
user 0m0.274s
sys 0m0.523s

I can see most of the time is spending on fetching the container image, I don't know why it has such a huge difference.