By default, the sliding is based on a percentage of the edge length for each vertex independently. The value goes from -1.0 (towards the vertex on one side of the edge) to 1.0 (towards the vertex on the other side), and 0.0 represents the original location.
Since the vertex on the right has a shorter edge compared to the other vertices, its movement will seem slower than the rest, but all of the vertices will reach the end of the edge at the same time.
Edge Slide has an alternative mode, Even, that will cause the sliding edge to take the shape of the edge loop before or after it. To enable Even mode, press E while the Edge Slide operator is active. To switch between the edge loops to match, press F (short for Flipped).
In summary, after triggering the Edge Slide operator, press E. If the edge is not shaped the way you want, press F.