Question about the carry chain of V generation deivces

Hi guys,

I read CycloneV&Arria V deivces handbook, there is comments "The two-bit carry select feature in Arria V devices halves the propagation delay of carry chains within the ALM" in LAB chapter. How to understant above comments? What's the "two-bit carry select feature"?

I think it compares with old devices families (for example Cyclone II, Cyclone III, Arria GX, Stratix II, and so on...), however, i still don't have any idea about the propagation delay of the carry chain inside other old devices? Can anyone give me such delay data?

Thanks in advacne!