A sheet of paper falls slower than the one crumpled into a ball since the sheet of paper has the larger surface area. The air introduces a resistance to the falling paper. Since the sheet of paper has more surface area than the crumpled paper, so sheet of paper experiences more air resistance than that of the crumpled paper.

If there was no air resistance (friction), then both the sheet of paper and crumpled ball of paper would have fallen at the same time.