Please read the legal definition of premeditation here:
What is premeditated and deliberate conduct?
Premeditation doesn't mean that someone planned out the crime perfectly and then carried it out perfectly. There is no legally prescribed minimum or maximum time necessary - premeditation can be accomplished in minutes or seconds (see the example at the link of someone simply deciding to pick up a hammer).
In the case of Libby and Abby, the victims were unpredictable as to walking speed, what they were doing, and even if they would cross the bridge all the way to the other side. So were any other potential victims that day. The killer may have hesitated in order to consider risk, or even the consequences of what he was about to do. None of that affects whether or not premeditation occurred. The fact that he didn't just run up and attack them right where they were but instead removed them to a separate location speaks to his taking the time to premeditate his actions IMO. I don't think the exact timing of the abduction compared to when he saw them is that significant in terms of analyzing what he's charged with doing.