Finer control over the speed of transition from one keyframe to another is a useful tool to have at one's disposal. Blender has its own editor that treats these transitions as a fully editable curve.
Manipulating curves to control the overall speed of an action, or control variations in speed across time is easy in the F-Curve editor.
1in the Timeline.
Cubeobject selected, move to the Texture panel and down to the Mapping tab.