Preserve horizontal speed on slopes

Hey guys I have a 3D character controller with rigidbody that moves along a grid. Basically, if you swipe right, the character will move right until he hits a wall. I also have gravity so my player can jump.

The issue is, when my player climbs a slope or stairs, he slows down drastically, because gravity is pulling him down. Is there a way to preserve my horizontal speed even on upward slopes?

Here is my movement code

 //If we are not moving...
             if (!isMoving)
             {
                 //If requested direction is valid, set it to currentDir and rotate character
                 if (CheckDirection(nextDirection))
                 {
                     currentDirection = nextDirection;
                     RotateCharacter(currentDirection);
                 }
                 //If we are stopped, calculate the next valid start and end position
                 startPosition = rb.position;
                 endPosition = startPosition + (currentDirection * unitLength);
                 //Round the result so we end up on a whole number coordinate
                 endPosition = new Vector3(Mathf.Round(endPosition.x), Mathf.Round(endPosition.y), Mathf.Round(endPosition.z));
                 isMoving = true;
             }
 
             //If we are moving and the current direction is valid (no walls in the way)
             if (isMoving && CheckDirection(currentDirection))
             {
                 //Move rigidbody towards destination
                 float step = speed * Time.deltaTime;
                 rb.MovePosition(Vector3.MoveTowards(rb.position, new Vector3(endPosition.x, rb.position.y, endPosition.z), step));
             }
             else
             {
                 RotateCharacter(nextDirection);
             }
 
             //Calculate horizontal distance remaining from startpoint to endpoint
             Vector3 distanceRemaining = new Vector3(endPosition.x, 0, endPosition.z) - new Vector3(rb.position.x, 0, rb.position.z);
 
             //If we are extremely close to endpoint, stop
             if (distanceRemaining.sqrMagnitude < float.Epsilon)
             {
                 isMoving = false;
             }
             //If we hit a wall, stop
             if (!CheckDirection(currentDirection))
             {
                 isMoving = false;
             }