I have discovered an annoying lag when typing on my Mac. It’s a fraction of a second, but it’s enough that it can be as much as a word behind.
The lag occurs when I use Firefox, Notes or Atom, so it is clearly not related to a single application.
I have recently upgraded to 10.13 High Sierra, as well as the supplementary update. I am running on a current model iMac.
As far as I am aware, I’m not running anything in the background which affects the keyboard. I even have background spelling turned off. And, of course, I have restarted.
This is not apparent on my MacBook which is similarly configured.