I have a 6s and XS non-Max and use both daily. A claim of 5% difference does not in any way match my experiences, and I say that as someone who prefers the 6S for its size and pre-FaceID UI. My XS consistently and handily outperforms my 6S (which does have a new battery, so it is not affected by battery-related performance downgrades).

The only difference between the devices is that the 6S does not have cellular service. Other than that distinction, all apps and settings on the devices are identical.

The only exception I see whereby the 6S is faster is with the Box.com app when it syncs the files I've marked for offline use. I have no idea if that's because of differences between the phones or something with the app.