Waiting for a couple of hours just to harvest a particular crops could make you get bored. There are only few things that you can do around your farm and one of it is planting and harvesting crops. If you really need to earn more coins then you might consider planting some crops that can be harvested after a short period of time. However crops that takes a little time to grow can be gone easily plus the fact that you can only get few coins by selling it in the market.
If you want your crops to grow faster so that you can harvest it, then you definitely need to have a Speed-grow to give a boost. Since Speed-grow can only obtain when you buy it using real cash, you might consider yourself very lucky because you are about to get not just one but three of it.