Hello,
wavelength refinement makes the mesh denser in areas with Er>1. If i'm not mistaken, this feature applies the "lines per wavevlength (LPW)" setting in the dielectric slab with the dielectric constant taken into consideration. For example, if your setting is 10 LPWs in an air brick (Er=1), then for a dielectric brick of Er=4 you'll have twice as many mesh cells for each direction - Meaning, that for a 3D brick the total mesh cells count will increase by a factor of 8.
BTW, for a more stable result I would try the energy based adaptive meshing.
P.