In unity, mouse presses accelerate roaming, and the mouse lifts up and slows down the roaming.

private bool Ismouseupordown=true; Start by default is mouse up state

if (Input.getmousebuttondown (1))//mouse pressed momentary state
{
Ismouseupordown = false; mouse button down
}
if (Input.getmousebutton (1))//mouse persistent pressed state
{
if (Ismouseupordown = = False)
{
if (movespeed >= 1.5f)
{
Debug.logerror ("acceleration");
}
Else
{
Movespeed + = 0.05f; Accelerated roaming
}
}
}
if (Input.getmousebuttonup (1))//mouse lifted momentary state
{
Ismouseupordown = true; Mouse Lift up
}
if (Ismouseupordown = = True)
{
if (movespeed <= 0.5f)
{
Debug.logerror ("deceleration");
}
Else
{
Movespeed-= 0.05f; Slow roaming
}
}

In unity, mouse presses accelerate roaming, and the mouse lifts up and slows down the roaming.