Question

Why does distilled water need very little soap to give lather

Answer 1

Soaps are actually sodium or potassium salts of long chained carboxylic acids. When they are mixed in water containing salts ( say those of Ca or Mg), they react to form a precipitate called scum. This stops the formation of lather. In distilled water, such salts are absent, thus allowing the soap to form lather.