There are always trade-offs in laptop design, and so while the MacBook is significantly more portable than the XPS 13, the MacBook does give up some processing power in return. While both the MacBook and XPS 13 now offer processors from Intel's latest generation—thanks to an update to the Mac lineup in June--the MacBook opts for low-power processors (from Intel’s "Y series," which don't require a fan). That means less horsepower; options are for Core m3 (clock speed of 1.2GHz), Core i5 (1.3GHz) or Core i7 (1.4GHz).
By contrast, the XPS 13 uses Intel's faster "U series" processors from the seventh generation—Core i3/i5/i7, with a base clock speed of 2.4GHz. In short, the XPS 13 is going to be noticeably speedier for most tasks.