@poe13: You had too heavy equipment on. Too heavy load makes your stamina/ki cost go through the roof and also shortens/slows the dash. I had the same issue on my second boss, didn't understand why my character couldn't swing so much. Spotted my weight load and just put on all light armor, like night and day difference. Putting on anything new with better defence you find in Nioh is a bad idea, since the best defence stats are from the heaviest gear.