I have a charging shelf on my wall unit. I keep three charging cords plugged in it all times. Two 5watt chargers (one with an old cable and one with the new lightning one). And one 10watt with an old cable on it.
Most of us are familiar with the little cube like 5W chargers and the larger 10W ones.
I rearranged my cables a few months ago and moved some things around. Since then my iPad3 has been charging extremely slow. I figured either my battery was going bad or my 10W charger. Today I happen to be in my garage and plugged my iPad into another 10W charger I have stationed in there and to my surprise it charged very quickly.
So I take out my charging shelf and take a look at all my cables to make sure I didn't mix something up. To my surprise when I took a close look at my 10W charger it didn't say 10W on it. It said iPod charger. And it also said 5volt 1amp meaning it was only a 5W charger. But it was the same size as a 10W charger.
I did a little google research and found out that Apple made large 5W chargers before going to the little cube ones. I'm not sure where I picked up the large 5W charger but I'm very glad I discovered the difference.
Here's the site that explains the differences of all the chargers in case anyone is interested.
Watt to Know About iPhone & iPad Power Adapters [UPDATED] - The Mac Observer