Hello,
I recently upgraded my mac to a new 2019 iMac and have been working in Scrivener with much delight.
Something that I encountered the other day when I wanted to limit my distractions and focus on the work is that when I go into composition mode, the words i type slow down. I mean that my typing is the same, but when I am watching the screen it is a whole two words behind me. I have to stop and wait for the type to catch up.
Thoughts?
UPDATE: removing the background image resolved the issue. I guess its the image.
I have noticed the same thing-- also on a new MacBook Air. I spent some time this afternoon setting up custom layouts so that I could have an almost distraction free view, because my screen was taking so long to catch up with my typing that it wasn’t possible to work. I don’t use a background, but I did have the background set to BLACK. After finding this thread, I tried typing with the opacity in the middle (almost black) and found it to be a little brisker. What I have noticed is that the longer I type, the longer it takes the screen to catch up.
I will try using composition mode tomorrow and see if adjusting the darkness of the background helps. But I hope the folks at L&L are seeing this thread, because it is definitely an issue for at least two people! 
I want to use a soothing photo background, but just like above, when I type normally, the words are way behind. I’d love to have this issue fixed so I can go back to writing in Composition Mode! TIA!