I just tried Virtualbox 5.2 for the 3rd time and for the 3rd time I went back to 5.1.
This time, I found this bug almost exactly describes my problem with VirtualBox 5.2: https://www.virtualbox.org/ticket/17573
The culprit is a huge difference with Host I/O cache. Here's my data doing a large update on an old 17.10 VM:
Version 5.2.12 HostI/O cache ON 51m46.641s
Version 5.2.14 HostI/O cache OFF 26m12.898s
Version 5.1.38 HostI/O cache ON 4m19.467s
Version 5.1.38 HostI/O cache OFF 26m 7.808s
You'll notice the two versions are virtually identical with Host I/O cache OFF but egad look at the difference with it on!!!
I have a theory Host I/O cache in 5.2 fails to cache downloads like 5.1 did. The result is extreme "unpacking" times during upgrading which has a large penalty for missing cache.
I don't want to generate an Oracle account just for this bug but welcome others to reference this post. I just hope it helps someone else.
Yeah, I know data corruption can occur with Host I/O cache with power failures and some resets of a VM but dang, the performance is worth it to me!