Human sized Diane might not be entirely faster, she'd just be more mobile/agile since she has a smaller frame. Her speed isn't apparent when she's a giant but it's clearer as a human. Asides that, I don't see how. Another issue is, what are we defining speed as here? Is it just how fast one can move from one area to another using whatever means or simply running speed? Because things could get confusing. For instance, Merlin can teleport at instantaneous speeds, making her the fastest to be able to move about. Her running speed is probably nothing to write home about and her reaction to attacks might not be that fantastic either. Diane threw Mel, King, Ban and Gowther all the way to Liones and they were a considerable distance away. It was certainly faster than any other sin could manage(asides Teleportation). If we use that as an example, she could probably gather enough power in her legs to leap much farther than the other sins can run, sort of like Galan did. Add that to her longer strides which could make a pretty big difference if she's running