Many Ballroom dancers (beginning and experienced) make an effort to move at the same speed throughout the dance when dancing Standard or Smooth. This seems to make sense because the music is "strict tempo" (the speed of the music is constant). This may be true if you aren't dancing with a partner. The leader must adjust the speed of his movements to accommodate the movement of the partnership (the two-headed four-legged animal). A two-legged animal moves very differently than a four-legged animal. The leader is the one that makes the two-headed four-legged animal a controlled and fluid moving entity.
For the partners to be moving at the same speed all of the time, they would have to be moving in a straight line, covering the same distance in the same amount of time. This would be a really boring dance with a really big dance floor. Once the partnership changes direction or turns, one of the partners is moving faster than the other. The individual dancer's body moves less and slower when that dancer is on inside of a turn. Bottom line, very seldom are both partners moving at the same speed.
The Ballroom dance partnership transitions from "dance position" to "dance position" as it moves around the floor. For example in Slow Waltz, the dancers will move from "Closed" dance position to "Promenade" dance position when executing a "Forward Hover" (Hover 1) element. They transition from "Promenade" to "Outside Partner" when dancing a "Hover 2" element and they transition from "Outside Partner" to "Promenade" when dancing a "Back Hover" (Hover 3) element. In all of the dance position transition elements, there is a speed difference between the partners as it is danced.
Many dancers seem to have the illusion that partners pass each other (go by each other) when they change from dance position to dance position. In reality, they just rotate around each other within the partnership. The dancers basically rotate around the center of the partnership. This is true whether you are dancing in "Closed" or "Open" partnership. If there is no dance position changes on the element, such as an American Style "Chasse" (starts in "Promenade" and ends in "Promenade") the speed of the dancers is the same.
The frame of the partnership defines the confines of the partner's movements ("Closed" or "Open" partnership). The frame is not frozen, ridged or restrictive. It is flexible and allows the partners freedom to move as necessary. The frame is a living, breathing part of the partnership. The Standard/Smooth frame is not stationary. The frame moves with the partnership and accommodates the dance position changes within it. Even though the dance frame is moving with the partnership, it is a fixed space for the dance position transitions to be executed.
When dancing Standard/Smooth, the ladies "belly button" is roughly lined up on the man's right hip bone (because of the partnership offset). This is a good reference point for maintaining the body of the "two-headed four-legged animal. When dancing a "forward Hover" the ladies "belly button" would rotate slightly around the man's right hip bone. The man's body would rotate slightly towards the lady. The lady will rotate with the man's body and then slightly farther to create the "promenade" dance position. This creates the correct body position.
From the leader's point of view, the "Back Hover" (Hover 3), transitions from "Backwards Outside Partner" (backing line of dance) to "Promenade" (facing diagonal center). The lady moves from in front of the man (O/P) to the other side of the man (Prom). If the man and woman move at the same speed, she will not be able to get to the other side of the man. The lady actually rotates around the right hip bone of the man to "Promenade Position". The problem is that the man is moving too. This means that the man has to move less and slower to enable the lady to pass to the other side. Generally speaking, the dancer moving backward has to move at a slower speed than the dancer moving forward.
I know this is hard to follow from a verbal description, but take the time to understand the concept of the partnership speed difference. This concept will change your partnership movement immensely. The best way to maintain the correct speed relationship is to simply maintain the offset position of each dancer in the partnership as they progress through the dance.