The easiest way to go faster in freestyle is for swimmers to change...

At slower speeds, with a front quadrant stroke, there are significant gaps in propulsion, where neither arm is creating propulsion.
More effort won’t solve the problem; they have to change their timing.
Even if swimmers are using the exact same effort and the exact same force, they’ll go faster because of the change in timing.
They need to create propulsive gaps to optimize efficiency, and they need to minimize them to optimize speed.
The right skills and the right strategies to help swimmers learn them.
The HOW matters, too.
Set design is critical, yet it’s more than that.