You feel like you can't think because your blood sugar is getting low. This stops as soon as your body flips over to ketosis (and apparently it has to 'flip' almost like a switch, the body is very bad at burning sugar and fat at the same time unfortunately.) Your brain cells can eat ketones just fine and you'll quickly feel MORE clearheaded / less brain fog once you get over the initial hump.

I only do intermittent fasting and low carb (never done a full multil-day fast) and even then I notice this effect on my thinking very clearly.