Generally speaking, faster should be sharper all other things being equal. The reason is that you are not likely to get motion blur at that speed. Blur from depth of field issues, however, is another matter. If the weather was excellent and lots of light I think I'd move up to f4, f5.6, or even f8 to give yourself more depth of field. If you had them running towards you or away from you it could be that you focused on them when they were at point A but captured the image when they were at point B and the result was out of focus because the depth of field was too narrow and they moved out of the focus zone before the image was captured.