What I don't get about this timescape theory, is what is the mechanism for time passing faster in the 'voids'? Or equivalently, what makes time pass slower alo

What I don't get about this timescape theory, is what is the mechanism for time passing faster in the 'voids'? Or equivalently, what makes time pass slower along the edges of the voids? Is it supposed to be gravitational time dilation? (e.g. 'low is slow') Surely this can't have an effect of this magnitude (a 2x speedup in the voids)? As I understand it gravitational time dilation only has significant effects (like the 2x slowdown) in the strong gravitational region regime, around black holes etc.