Ironically, Ruby has lost time and time again because she wasn't fast enough. She couldn't capture Torchwick, couldn't stop Cinder, failed to save Penny couldn't stop the train, failed to beat Torchwick AGAIN, and wasn't fast enough to stop Pyrrha's death. So far, the only thing that speed has won her is a food fight.
Well she is just a begginger at being a huntress, and Torchwick, along with Cinder have been doing this for a longer than she has been with her sythe, aura and semblance. True she did fail to pass Mercury to get to Penny and Pyrrah to stop the fight, failed to save Pyrrah from Cinder. But you can't really blame her for being late, sure she has speed as her semblance, but sometimes speed is not that easy to control.