Glacial is frequently used in just this way.
"suggestive of the very slow movement of glaciers ·progress on the bill has been glacial"(MW)
Interestingly, there is a concept in fictional narrative that may also be useful. If a long segment of the text is devoted to a short period of the story, it is called deceleration. So a decelerated event has a short duration in time but extends for an inordinate amount of text compared to the normal event time/text length for the narrative. Narrative Fiction, Shlomith Rimmon-Kenan, Routledge.
In more general use, "The event decelerated as the week progressed."
The time we spent on the island has decelerated. It is as if we'd lived there for years.