Strange the way that works.
I notice the same thing with audiobooks.
I can speed read/listen, but I have to let my brain adapt for about 5 minutes.
What I usually do is ease into a higher speed slowly or start out fast and then go back to the starting point once my brain has adapted.
I can cover much more material that way.;-)
I can relate, I am like that until I get a couple cups of coffee in me... :)