It took a while for me to start burning fat efficiently when I started keto. YMMV. Day 1 was ok, 2 was difficult, 3 was the worst - had a persistent headache and my mind was at maybe 50% of what it is normally. After that I was fine, except for two issues -

1. I didn't feel hungry anymore, so I didn't want to eat. I had to force myself to eat protein so I wouldn't lose muscle.

2. I couldn't do strenuous exercise for at least a week afterwards. I could walk for several km at a time but running was difficult. Running up stairs even more so. But once I was fully adjusted to the diet 7-10 days in, I could do all of those things.

I'll return to this diet later this year. Really looking forward to it.

What I described happens after a few days, maybe up to a week for me. Right after getting really thirsty and drinking a crapload of water and pissing it out (I presume this is for hydrolyzing glycogen...) for 1-2 days.

Like I said, I already wasn't a big consumer of carbs/sugar to begin with though.