contrast slider adjustment timeout/lag

[ngreenwald]

The dynamic Vmin/Vmax adjustment freezes and goes quite slow on large images. Visually, it looks like it is going through all of the possible values between the old slider setting and the new slider setting sequentially after each adjustment. Some of the other sliders appear to have the same issue, but the vmin/max is the most extreme.

Perhaps changing it to a textbox where users could enter the new value, rather than a slider, would resolve this issue? Or it could be something specific to how the slider is implemented.