They seem to be C3000 (I'm guessing the old models will get switched over to C3000 soon-ish): https://www.ixsystems.com/freenas-mini/ (click on full specifications)
Mini E has a C3338 and the Mini XL+ has a C3758. I'm not sure anybody on the forums actually tried the dual-core one, but I expect to be similar to a C2550, with the much-improved microarchitecture making up for the lack of the extra two cores. C3758 should be pretty neat.