Why does time “seem” to slow down when one is bored, versus the sensation of time moving quite fast when one is engrossed in a rewarding activity?

This is an interesting paradox, is it not? The differing perspectives are a consequence of the workings of consciousness. You are designed to seek change, growth, novelty, and things that will bring a reward of some kind, a positive experience, an expansion so that your quest can continue and ideally, be never-ending. This is why having a limited life is so stultifying and boredom so dreadful and even deadly, because you, in that state of mind and environment it brings about, become blinded to the great wealth of opportunities and have suppressed the vision of things much greater and much grander that are your birthright and heritage as a divine extension. Your potential is vaster than you know, and that makes a difference in what you can perceive, but that is part of your journey and your experiencing as well. Otherwise, there would be no novelty, no delight, no excitement, and no challenges if everything were predetermined, known in advance so that everything could be planned for and would, in effect, already be secondhand when it comes about, because it had been foreseen and in a kind of dress rehearsal, so it becomes an anticlimax with the unfolding of the current reality. So to be all-knowing and to have foreknowledge is a blessing but it is not a tool to be embraced in how to live one’s life from day-to-day on a constant basis. You must do the best you can with what you have and it is that challenge that is giving you seasoning, a toughening up, and a claim to fame in the bargain.