Lxc export slow

How long is lxc export meant to take on average using dir?

From an ubuntu image with no additions, running the following is taking a long while and the resulting tarball is only 98M, I know cpu/disk spec makes a difference but looking at top, xz hammers a single core.

root@server:~# time lxc export ct --container-only --optimized-storage
Backup exported successfully!           

real	4m33.762s
user	0m0.028s
sys	0m0.199s

Resulting backup size:

-rw-r--r-- 1 root root 101831860 Jun 19 15:53 backup.tar.xz

Is this normal?

The flag --optimized-storage is probably doing a very good compression. I do not think the compression is multithreaded (so that it gets split into several CPU cores).

Regarding the size, which exactly container did you start with? You can do, for example, lxc image info ubuntu:16.04 to get the container image size.
Still, 98MB for any Ubuntu image is quite small because the container images are around 140-160MB.
Perhaps someone else knows a bit more on this.