Fast set joint compound taking DAYS to dry?
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 &quot;vintage&quot; scrape it off and get some new compound from a sealed container.