There is only so much to be had in a given technology
because in attempting to reduce R, you must add C.
You can play some tricks like using two-finger devices
and driving the outer fingers with the single inner, going
to the summing node, and so on. But you may want to
look at other ways of steering current, such as "source
steering" in the current mirrors themselves, and lose
the "baggage" of transmission gates in series with fixed
current sources.
If this is what's going on... as usual, no schematics,
just "gimme answer".