Same here. I have a small theory that this is some kind of natural mechanism that balances out the pure hyperfocus bliss that you experience when doing something you like to do.
I don't know if this is the case for you too, but for me this is the worst when I am made to do something I dislike doing. What's worse is that I spend enormous amounts of energy focusing on how much I dislike doing this(!) And with every passing minute, the task seems more daunting.
Sometimes, when I finally get started, it's not too bad (though in my head there's still a small storm raging), but on other times it even takes me hours to get the smallest tasks done
This is a very real problem at work; I'm extremely good at what I do, so it becomes more glaring (at least to myself) how incredibly long it takes me to do these boring tasks which should take mere minutes...
Sometimes I'm afraid my boss will notice my slacking off so extremely and get angry. Fortunately this hasn't happened yet!
You described it pretty accurately. That's exactly how I feel and what happens to me, specially the "I spend enormous amounts of energy focusing on how much I dislike doing this" part.