When the UE has obtained the slot after decoding PSS, the best case scenario is the UE figured out the Frame boundary(using SSS) in Subframe 0(which happens in slot0), in which case it can immediately(in the slot1 of subframe 0) obtain the MIB info. The worst case (or the only other) scenario is that it figures out in the subframe 5, in which case it has to wait another 5 subframes for the Frame start.
Once it obtained the Frame boundary, it makes sense to finish the MIB decoding in the subframe 0 itself, rather than waiting for another 5ms.