From my poor understanding and I could be totally off… it’s from the expansion of the universe itself. Like the doppler effect with a siren, as it moves past it sounds longer as the Soundwaves are stretched moving away from you, the same thing happens with light as the universe expands. Here they are looking at quasars which they know “blink” at a certain rates, and from my understanding which is limited and could be totally wrong, that blinking is also stretched out causing it to appear slower.