I've made two changes affecting killspeeds on your list: lethality and mental stats.

I gave every unit either a 3,4,5, or 6 point mental boost. Basic untrained units got a +3, trained or those better than militia got a +4 bonus, elite units got a +5 and the super elite and advanced generals got +6. This had led to battle lines that don't fold instantly. I'm considering adding another 1 to all units because even now a cavalry charge can reduce a unit's morale to breaking point too fast. However I don't want to add too much morale: units should break if they're losing badly. It's just a shame when they rout they die so easily. I think that's where much of the battle length problems lie.

Regarding lethality, this is a good one to change. Something I will note first, even before changing lethality, the mental boosts can make battles last long. I had a 1 Auxiliary unit versus 1 Auxiliary unit battle before making changes to lethality. The battle lasted six and a half minutes. I refought the battle and the next one lasted over 10 minutes! That's 242 men in total (121 on each side inc. captain) fighting for over ten minutes! I had to use the 'Rally Troops' command twice to keep the men fighting for so long, and in the end I had only 4 men left before they routed. Reducing their lethality would in theory lead to silly lengths of battles.

However, it had to be done, especially when considering often several units are engaging each other at once - it's not always spearmen versus spearmen. I reduced most units lethality by around 0.20-0.27 and this makes the fights in the game last a bit longer. With the mental stat boost, I even see the AI will sometimes pull out it's wounded men, then charge them in again, whereas before they would probably have routed before getting the opportunity.

Still, cavalry and missiles are still overpowered and can turn potentially long battles into nothing more than short skirmishes. My next step is to reduce the number of cavalry in a unit and try and figure a good way to reduce the lethality of missiles.

I've considered upping the hitpoints as well, but I don't want unnecessary long and tedious slug-fests where nothing much happens for a while. Plus I'm not sure if the battles are slowed down by too much whether the AI would benefit more, or myself. I don't need any more advantages over the AI.