The term 'REALLY FAST' can be summarized with three crossword solutions: PRESTO (6 letters), SUPERSONIC (10 letters), and LIKEAHOUSEAFIRE (15 letters). PRESTO is an Italian word meaning 'QUICKLY' or 'IMMEDIATELY'. SUPERSONIC refers to the speed of sound, which is faster than the speed of light. LIKEAHOUSEAFIRE is an idiom that means something is happening very QUICKLY, as if it were a fire spreading rapidly. All three words describe something that is happening at a very fast rate.