Fortunately, there's only so fast that a fire can rise when you don't have fire magic to nudge it along. So Sans doesn't do anything to try and hasten her along. He's genuinely not sure that he could, at this point. Instead, he just follows her into the kitchen at a shuffle. When Grune moves to the fire, Sans moves to the table. He takes a seat, folds his arms on top of it, and slumps forward to rest his head on them.

So his voice, when he speaks, is a mumble. But it's a heartfelt one. "thanks for this, grune."