You can try using an object pool. So at the start of the game, create 20 coins (whatever the maximum that will ever be shown on screen at once), but hide them. Then when you want to spawn a coin, just show it (and switch the color) and move it the the right position. The slowdown is usually caused by creating objects. This is commonly done in 2D shooting games, where there are lots of ships or bullets, as it will be too slow to constantly create/destroy that many objects.
