Uncover mini/world map

Hi all!

I like to see if I can get any tips on how to make a mini/world map that the player can uncover after time. Like many games now have (Assassin's Creed and Far Cry is just some I can think of).

So far I have made all these small balls and placed they in front of a top down orthographic camera and made a script to remove those over the player. This worked greate until I made the balls smaller and therefore more balls, but that caused lag (I don't know what I expected).

If anyone knows a great way of doing this pleas tell me.

//Netråm