Through some helpful members on the Avid DUC forum, I have narrowed down a crippling GUI lag issue to being caused by having VEP 6 instances open in a moderately large (200+) MIDI session in Pro Tools 2018.1. (latest version.)
This issue does not occur in PT 11.3.2, which I had to downgrade to in order to finish a project.
My main PT system:
Mac Pro 6,1, OS Sierra, PT 2018.1, VEP6.
2 x Apple Thunderbolt 27" displays.