Why does the cascode current mirror have a lower output capacitance?
The effective capacitance as seen at the drain of Q2 is enhanced by the Miller effect. A positive voltage transient applied to the drain induces an increase in gate voltage (since the transient is conducted through Cgd), which induces an increase in drain current--the effective capacitance is thus increased.
In the cascode arrangement, Q3 acts to isolate Q2's drain from voltage transients at the output. A transient applied to Q3's drain would result in only a tiny voltage transient at Q2's drain (since the Q2 drain voltage is effectively regulated to stay one Vgs drop below the constant voltage applied to Q3 gate). Another way to look at this is that Q3 acts as a current buffer--Q2 outputs a constant current and Q3 buffers it to be resilient to output voltage transients. Because a transient voltage applied to the output no longer leads to a significant transient current, the effective output capacitance is reduced.