1. How did you setup the instance variable for sprite2 for it to show as <varies>
You can set instance variables individual for each by click on the instance in the layout and set it. If you click then in the Objects-Windows of the one object it will show no value (or varies in the add instance variable window), because the instances have different values.
2. Would the speed of moving sprite vary according to the performance of the target machine?
Yes it would, you can prevent this if you use delta time. In this example: 'Move Forward 60*dt'.
This uses delta time 'dt' which is the length between the ticks (normally 1/60 sec for 60 frames per second).
If your game slows down the length between the ticks will grow. So, it will prevent that it will goes slower but it will go choppier. And if it goes to slow you can get problems with collision, because if your sprite does every tick a hop of 20 pixel it can miss the trigger for the collision and go through a wall or enemy.