It is a power PC triple core CPU. They would not have changed the power of the CPU in the slim as it has to be the same as the original 360, they have just improved the design and merged the GPU with the CPU.
As noted, nothing has changed. The performance of the XCGPU in the 360 S is the same as the original Xenon CPU in the original 360. Anyone who tells you differently has not read up on what IBM did to ensure moving the CPU and GPU onto the same die would
not result in a performance difference.