The pace of real-estate speculation is slowing, a sign of the darkening outlook in the U.S. housing market. It was the smallest amount since the second quarter of 2015, according to Attom Data Solutions, a real-estate data firm. Higher interest rates have been putting a damper on the U.S. housing market, a key building block of an otherwise solid economy. The fix-and-flip crowd, numbering some 38,000, has been on the front lines of this shift. Fewer than 46,000 single-family houses and condos were flipped in the third quarter, the smallest number in three-and-a-half years, according to Attom.