Try this and see if it works: Edit the export_descr_unit.txt file
and search for the archer unit you are interested in. Find this
line

Code:
stat_pri         6, 1, arrow, 120, 25, missile, missile_mechanical, piercing, none, 25, 1
The comments at the top of this file say that the next to last entry,
the one I colored blue, is the time between attacks in tenths of a
second. So if this works for missile units 25 means they shoot every
2.5 seconds. Try changing this to 100 and see if they slow down
a lot. Haven't tried this myself so its only a suggestion.