If by "zone boarding" you mean window seat first, then I am inclined to
agree,as the delays caused by people putting the baggage in the overhead
has little, if anything. to do with where people sit in a row.

I have been on AA flights which used a similar zone boarding, and
my experience is that this is an improvement to the usual by-rows
procedure.

If the airlines want to make a noticable improvement in boarding times
they should consider boarding from both front and rear. Which I am aware
will require some changes to the current single jetway arrangement. Those
of you who took flights prior to jetways might recall that it was common to
board and exit from stairs on both the front and rear of the aircraft.