Change speed over time

You could use a simple countdown variable to see when it's time to change the speed

 const float SPEED_INCREASE_INTERVAL = 3f; // how often do you want the speed to change
 const float SPEED_CHANGE = 1; // how much do you want the speed to change 
 float _ItervalTimer = SPEED_INCREASE_INTERVAL;
 
 void FixedUpdate () 
 {
    if (isBackground == false){
        _ItervalTimer -= Time.deltaTime;
        if (_IntervalTimer < 0)
        {
            speed -= SPEED_CHANGE;
  
            Vector2 velocity = new Vector2(speed, 0);
            rigidbody2D.velocity = velocity;
  
            Destroy(gameObject,destroyTime);
            _IntervalTimer = SPEED_INCREASE_INTERVAL;
        }
 
    }
 }

EDIT: To keep spawning objects with increased moving speeds as requested:

You need to have a script that stores the time when the level started. This timestamp needs to be captured before every time you enter a level to play it (don't put this into every GameObject. Put it in just one)

     public static float _TimeSinceStart;
 
     public void StartGame() // call this every time the player enters a level to play it
     {
         _TimeSinceStart = Time.realtimeSinceStartup;
     }

Then you can spawn objects that move faster depending on how long the level has been played:

 const float SPEED_INCREASE_INTERVAL = 15f; // how often do you want the speed to change
     const float SPEED_CHANGE = -1; // how much do you want the speed to change
     const float SPEED_AT_START = -2;
 
     void FixedUpdate () 
     {
         if (isBackground == false)
         {
             // how much time has passed since game started
             int secondsSinceGameStart = (int)(Time.realtimeSinceStartup - Game._TimeSinceStart);
 
             // how many times has the speed changing interval passed
             int interval = secondsSinceGameStart / SPEED_INCREASE_INTERVAL;
 
             // current speed = start speed + speed increase for every interval that has passed 
             speed -= SPEED_AT_START + (interval * SPEED_CHANGE);
             
             Vector2 velocity = new Vector2(speed, 0);
             rigidbody2D.velocity = velocity;
             
             Destroy(gameObject, destroyTime);
         }
     }