I have a part that consists of two surfaces connected by a boundary surface and thickened.   I put 270 of them into a sub-assembly, then inserted that assembly into another assembly.  The main assembly is now so slow that it is almost impossible to work with.  I made the sub-assembly lightweight and it doesn't seem to help much, but if I suppress it then everything is OK.  I don't work with surfaces much, but would that type of part usually cause that problem?  Sorry, but I can't post the assembly.