Just thinking outloud…
It sure sounds like graphics but I was looking at what ubuntustudio-desktop adds. A Plymouth theme caught my eye. That is consistent with up AND down but not login to the desktop.
What happens with a Console Mode boot?
I wonder if systemd-analyze blame might provide a direction. If it hangs everything, then everything may just show high times making it useless.
The latest ISO, and this is relatively new to this LTS, uses the hwe (Hardware Enablement) kernel so the LTS tries to keep up on new hardware but I’m not convinced it solves more problems than it causes. You probably have the 4.10 kernel while an original ISO uses 4.4. You could test the 4.4 kernel.
EDIT: I just tried a VM installed with 16.04.3 ISO (4.10 kernel). One meta package linux-generic pulls in the entre 4.4 kernel but you have to explicitely select it in grub. I didn’t see where UbuntuStudio installs real-time or low-latency kernels - thought it did.