Is my graphics card the bottleneck slowing down the pressure sensitivity on my graphics tablet?
OK, so after upgrading my Macbook to something with much more horsepower the problem still existed, but to a lesser degree. I also updated Illustrator, but unfortunately couldn't roll back to CS6. 

The issue was lesser, but still a major problem. I just bought a similarly priced Wacom Intuos, and the experience is better in every way. No lag, perfect pressure sensitivity, and no sensation of long brush strokes becoming increasingly angular as they develop. Likely a poorly written driver from Huion - I would often find that pressure sensitivity would drop off the grid entirely until I restarted the driver process.

Moral of the story is not to buy Huion, unfortunately.