Don't use LINQ?

LINQ is slow, in general, but the fundamental solution is to reconsider the problem. I would guess that the "1" you are using is just an example, or else there isn't any point in the LINQ, but one thing you might consider is putting the tblStructure into a Dictionary with the parent ID as the key. Finding out whether or not a Dicitonary holds a key is VERY fast compared to what you are doing. Whether or not this will work in your case is harder to say.