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 dehumidifer on but didn't get a chance to put in a fan/heater yet. the day it dried it was raining a lot ironically
Best Answer
Two possibilities:
There is too much humidity in your building. This can often be the case with new construction in humid areas. It can take many days for the HVAC system to remove enough humidity. In existing construction you still may have excess humidity depending on the location, such as a bathroom. You might find a dehumidifier helpful.
The compound is old. In this case it may never dry. If you don't know its "vintage" scrape it off and get some new compound from a sealed container.