We weren't there in the cockpit to know what the PF was intending, or why he flew so slowly. However I would disagree that flying at a higher speed would have caused more deaths. Rather I would argue just the opposite.
A higher speed would have improved the glide angle, and so have given more clearance over the obstacles, and a greater range of landing spots in the river. A higher speed would also have given more kinetic energy for better control in the final few seconds of flight and allowed for a much gentler touch down into the water. (yes, that was their only option).
Instead it appears that the PF was trying to increase time in the air at the cost of energy and distance. Maybe he wanted more time to restart an engine. We will just have to wait for more information before we can ever know the answers to these questions.