Whenever you animate with key frames, even though the speed can vary, it's always only a straight line, so the end result usually looks jagged in some way. Is there a way to make an animation keyframe from point A to point B make the shape of some kind of subtle arc rather than just a straight line?
Use the Graph Editor to edit the animation curves. This will allow you to have full control over the shape of the animation. In the graph editor you can select a specific channel (such as the Z Location) and can then add keyframes by way of the Key/Insert Keyframe menu (or I shortcut) and these can be manipulated by grabbing and rotating the bezier handles (assuming it's set to Bezier - the default interpolation mode).
Once you've adjusted the curve it is useful to use the Motion Paths panel to visualise the new path (as in LeoNas's answer).