it depends. you
could mimic the formula logic in a stored proc, it could also be the number of rows, how many tables are involved, joins, etc. If it is a stored proc, how long does it take to run on the server? If the tables are heavily used and there is a lot of filtering the tables could be getting locked. How long does it take to run on someone else's machine?
Obviously, there are a whole host of issues that could be happening.... and there just isn't enough info to point in any direction.
HTH