I hit the same issue as you. It seems a fairly rare use case, as this is pretty much the only result that comes up for it on Google.
I did eventually find the issue though, and it is to do with mounting /proc into the chroot (from within the Docker container)
In your example, prior to entering/using the rootfs you should mount Docker's /proc into the chroot:
mount -t proc /proc <armbian-rootfs-dir>/proc You will then get native speeds for cryptography within your chroot
Edit: This appears to have worked fine on an old Ubuntu 16.04 image chroot, but fails to fix the issue when using debootstrap.
Looking at the contents of the chroot created by debootstrap, the proc folder appears to be a symlink to /proc. My understanding is this should not work, and indeed, running ls /proc from within the chroot results in cannot access 'proc': Too many levels of symbolic links
To get around this (and I'm sure someone will tell me why this is a bad idea), from outside the chroot run:
rm <armbian-rootfs-dir>/proc mkdir -p <armbian-rootfs-dir>/proc mount -t proc /proc <armbian-rootfs-dir>/proc This removes the symlink and bind mounts the chroot's /proc to Docker's /proc
It is also worth noting that debootstrap --second-stage appears to unmount \proc/proc again, so you will need to remount it (from outside the chroot) when that phase has completed (ref: Accessing /proc from foreign chroot)
Note: Don't forget to umount <armbian-rootfs-dir>/proc when you have finished with the chroot and wish to image the folder.