From VFXPedia

Time Stretching

Time Stretching allows for proportionally stretched or squashed selected splines, or segments on that spline. To enable spline stretching, select this option from the mode submenu in the context menu or click on the Time Stretching button in the toolbar.

If more than one keyframe is selected on the spline, the outer keyframes will be surrounded by two vertical purple bars. If no keyframes are selected, click-drag a rectangle to set the boundaries of the bars.

To disable the time stretching mode, click on the toolbar button again or reselect Modes>Time Stretching from the context menu.

Click-drag on the purple vertical bars to stretch or shrink the timing of the spline segments within the bars. Drag these bars back and forth to stretch or squash the spline segment in time.