In TFA after the Garrison discovered Kelsier's army, Kelsier asks Sazed how much speed he has stored up. Sazed answers that speed "is a very difficult attribute to collect". That made me think. Why is it so hard to store huge amounts of speed in a steelmind? You could simply slow your body completely down while lying in bed before you fall asleep whitch would result in very much stored speed every night.
So there must be a hitch with it. What if storing speed not only slows down your movements, but the whole functionality of your body? That would mean alongside witch your movements your heartbeat, breathing etc. would be slower, too. So by storing too much speed you would simply kill yourself because your vital bodily functions would be so slow as if they have failed.
This is the only explanation I could come up witch. Your thoughts?