Registering multiple keypresses simultaneously, very slow
What is impacting you performance so heavily is the creation of Tuples everytime you recieve a keypressed event.

creating objects is slow. What you should do instead is, directly apply the changes to the moving objects (your two pong-thingies).

private void GameForm_KeyDown(object sender, KeyEventArgs e)
{
   var pressedKeys = inputKeys.Where(key =&gt; Keyboard.GetKeyState(key));
   foreach(Keys key in pressedKeys)
      switch(key){
         case Keys.Up:
             leftPong.Positon += FIXEDVALUE;
             break;
         case Keys.Down:
             leftPong.Position -= FIXEDVALUE;
             break;
         //continue...