Adding a "Speed" control to a pan

I wanted to create a slider that would control the speed of my panning bg.

So far, I created a position control ("StartPos") and a slider
("Speed"). Position expression =
StartPosX = effect ("StartPos")("Point")[0];
Speed = effect("Speed")("Slider")*(time*24);
x = StartPosX - Speed;
[x, 243]

This works fine, until I want to animate the speed changing (0 -> 4 to start
it panning, and then 4 -> 0 to stop it)