Yeah, I see what it is. . . if you have lots of fill cells, then every fill cell is permutable with every other fill cell, so they all represent unresolvable symmetries that require arbritrary breaking. Netgen is doing an N^2 loop to check whether any of them have properties that would distinguish them from one another. Of course they don't have any properties, but just checking all those cells against each other takes forever. So I'm pretty sure my solution is sound, to flag designs where components don't have properties and then just skip the property checks for symmetry breaking. Problem solved.