You're right! I ran the tests in Debug mode on a 17-inch Late 2011 MacBook Pro with a 2.4 GHz Intel Core i7, being held together by duct tape (don't ask), in Xcode Version 10.0 beta (10L176w).
Here are my results using Release (-Os):
x1: 0.131 s
x2: 0.117 s
x3: 0.120 s
x4: 0.120 s
x5: 0.121 s
x6: 0.193 s
Very peculiar. 