You can make it based on how many units its dragging.
For example if it drags one unit, its speed would be faster rather than dragging five.
Like "Every Number of Units * 0.1 seconds"
Also you can check how far the unit is from the missile. If its more than X range, let's say 300 it will be dragged slower. The more its pulled towards the missile, the faster it gets.
Just my two cents on the matter.