Before revision:  In discontinuous Galerkin time-domain (DGTD) methods, assigning smooth-varying coefficients calls for storing individual mass matrices for each mesh element , which significantly increases the memory-cost.
After revision:  In discontinuous Galerkin time-domain (DGTD) methods, using a PML coefficient that varies within a mesh element requires a different mass matrix to be stored for every element and therefore significantly increases the memory-cost.