I'm not sure you can talk about whether TW kill rates are "historical" in any literal sense[1]. The TW battle engine, while one of the best available, is too stylised to be taken as a direct representation. What actually went on in a battle - the orders, the pauses, the confusions, the fog of war etc - is light years away from the simple clash of units in the game. There's also an element of scaling - most real battles lasted much longer than we want to be spending playing them out and often involved a lot more men than our computers can handle. (RTW probably works best if you see it as 1 computer man = 10 real men.)

To me, the issue of kill rates, like movement speeds, is primarily a gameplay one - if they are too high, the action is too frenetic and hard to control. Slowing things down allows you to use tactics more and also to savour the action.

[1]If we were to take it literally, RTW kill rates are not necessarily absurd. I remember seeing a medieval warfare expert on TV with some of his students. He got one, a fit looking young man in armour to engage in sequential combat with half a dozen of the others. He was exhausted after 20 seconds.