This, on its own, is not a sign of ADHD and can be explained by other mechanisms.

Here are a few alternative mechanisms:

• Maxing out your cognitive load. We can only think so much, so if you think beyond your capabilities, you get overwhelmed and can’t think well. This happens to everyone, regardless of whether they have ADHD or not. Check out Sweller’s cognitive load theory. He has some talks online.
• Adenosine buildup. This is a molecule that gets produced while you think. Your brain slowly gets filled with this molecule and it is one of the ways in which you end up feeling tired. You can actually trick your brain into thinking there’s no adenosine; you can drink coffee! Coffee fills your brain with caffeine, which crowds out adenosine. Caffeine, over time, will leave your body, but adenosine will only leave when you rest. That is why reading can feel difficult late at night and easy after a good night’s sleep.