On further investigation, Arne's absolutely right. I upgraded the kernel back to 4.9.88-1 from Debian Security and installed 'haveged' (another random number generator). Everything started quickly and normally after a reboot. Turns out I hadn't noticed this on any of my other virtual servers because they're all running haveged anyway.
So, Workarounds: ============ 1. Roll back kernel to 4.9.82-1+deb9u3 OR 2. Install another RNG, such has 'haveged'
