Some ideas (considering that tacical combat should stay fast):
1) The minimum you should do for placing the units on start is a heuristic that moves ranged and "weak" melee (mages, pinoneers) units in the second row and strong melee units in the first row (you just have to "look" at the weapons).
2) It is sometimes possible to do something on the strategic map while the tactical map gets loaded. That should not be possible.
3) Wounding a unit that prepares a 2-round-spell should cancel the spell
4) To do 3) you probably should implement basic "line-of-sight" so that hitting units behind other units gets more difficult
5) Did you consider invisible/hiding units?
6) I think the damage value of critical hits sometims is too high (killing a well armored, high hp hero with one strike is extreme)
7) Did you play Diciples 3 and know the simple "key terrain"-concept? I think something like that could make the combat more interesting without causing delay.