This seems very unusual. Fast set joint compound, in my experience, will even set (on the inside anyway) under water, as my buckets have proven.
Yet I needed a hearty amount to plug a deep hole, and it's literally been 4 days and the surface is still wet to touch, and visually you can tell it's still wet (though the dry part is slowly growing day by day).
What could be causing this? (Should I be worried about effects later on. E.G. after priming & painting)
UPDATE: It dried after 1 week, I had a dehumidifier on but didn't get a chance to put in a fan/heater yet. The day it dried it was raining a lot, ironically.