Slow down a character while they are midair while keeping their original velocity

I want my character to be able to jump in the air but I don't want them to move as fast while in the air except in the direction that they were moving when they jumped. I had the idea of making all of their movement inputs count for half when in the air and that works fine for slowing them in all directions but they get slowed in the direction that they were moving when they first jumped.

   void PerformMovement()
   {
       if (velocity != Vector3.zero)
       {
           if (isFalling)
           {
               rb.MovePosition(transform.position + velocity * Time.fixedDeltaTime / 2);
           }
           else
           {
               rb.MovePosition(transform.position + velocity * Time.fixedDeltaTime);
           }
       }
   }

How can I keep them moving in their initial direction while still allowing them to make slight adjustments while in midair?