if it's only 2 keyframes, changing the interpolation without extracting the handles will not change speed or the motion path. if you do extract the motion keyframe handles, or add more keyframes that are not on a straight line, then you can affect speed and motion path because the element is having more distance between keyframes. but no affect on acceleration or deceleration - affecting speed using the motion path is limited.

if you want to really affect speed - acceleration or deceleration. you need to affect the temporal interpolation as well. things will be more clear if you examine the graph editor and watch the speed graph. here's how it looks when I extract the handles in the graph editor.

for the most control though, you can affect the spatial interpolation as well as temporal interpolation at the same time, then use the value graph but you need to separate dimensions first for position transform
