X-Range offers wider range, more useful and smoother gear progression, as well as smarter shift settings - enabled by AXS.
The 12-speed cassettes move some of the gear range from the chainrings to the cassette, and provide tighter jumps between cogs. The 10T smallest cog and the possibility to ride a 36T largest cog (360% range!) allow the rider to stay on the same chainring longer, requiring fewer front shifts, that are inherently less efficient.
2x setups consistently offer a 13T difference between chainrings. The small jump allows the rider to easily find the right cadence, with fewer compensation in the rear.