After much playing around with VBox 5.2 settings, additions and drivers - all my Ubuntu and Debian VMs take 10 times longer to unpack kernels (I believe kernels are just the biggest files I encounter like this). I've also manually downloaded and extracted the exact deb files giving me fits - no issue with this method.
Searching around the boards makes me think I'm the only one in the world seeing this.
So I looked for a simple command and ended up with this as a benchmark:
time sudo apt install --reinstall linux-image-extra-4.4.0-104-generic
This happens to be for Ubuntu-MATE 16.04 original 4.4 kernel but you can change to your latest linux-image-extra kernel which is particularly big and slow. And while I didn't keep benchmarks for all, other versions show the same slowness for me. The regular image takes half as long.
What do you see? 20 seconds or 4 minutes to reinstall your latest linux-image-extra?
EDIT: Just tried 18.04 daily and get over 5 minutes (5:15) but 30 seconds is building guest additions. The time is dominated by unpacking. The package used is linux-image-extra-4.13.0-17-generic.