Actually, you can turn the finishers off in the game options.
Regarding "Horse Slow-Mo" - it is not meant as a sort of finisher. On the horse, when you hold the button for fast attack, it slows down time when you are close
to an enemy. This allows you to line up your attack a bit better for a few split seconds. The window for this is relatively short, as it sort of wears off as soon as you get too close to the enemy.
Consider this: horse combat without the slow down didn't play too well, since the horse movement is quite fast. The high speed, when testing, got even more problematic on horse to horse combat, so the short slow down window (again, only activated when holding the button within a certain range of an enemy) helps a lot here.
Edit: Typoooos the german in me couldn't resist to fix.