[8 Hour Whore]

Incoming wisdom from one handed fc Masta!

Using one handed on arrow keys or whatever set up you are using is always going to have its disadvantages.
Going from down arrow to up seems simple enough. The catch is when you are going from up to down you can simply slide your finger or quickly tap it due to the natural curve your fingers do.
When going from down to up you have to extend your finger rapidly and to do so it isn't always how you hoped.
My suggestion is get a flat keyboard and try learning to slide on flat keys. Make sure the keyboard won't catch on your fingers when sliding up. It helps incredibly in those up down up down trills. I support your one handed play but I warn you it is incredibly risky. Wrist problems come quicker then switching to spread.