I recently switched from 8 -> 9 and can't help but notice that moving the focus circle in the plex is much laggier than it is in 8. There's a 200-700ms lag before the focus indicator begins to move from one thought to another.
Looking at Activity Monitor I see that the resting CPU usage with The Brain active, but not being used, is ~11%, then when I move focus, it jumps to ~30-50%. Both seem too high and the performance while moving around the plex is distractingly slow.
I'm using High Sierra on a 2016 MBP.