Why are some USB chargers slower than others? It seems with USB chargers for my android phone, not all are created equal.  

The first pic is the official charger which came with my phone (galaxy nexus).  It charges quickly, from 0 to 100% in a couple of hours max.  

The second pic is a cheapy I got on ebay to use at the office.  It works but it's slow as a wet week, can take 6+ hours to charge and if I use the phone while it's charging, it seems to drain faster than it's being charged.  

The two chargers below seem to be the same spec (5V / 1A) and from my limited understanding that P = V*I they should provide the same power, right?

What else do I need to look for when buying a charger, and is there any way to tell in advance if it is going to be slow or fast (apart from testing them out)?

 
(click images for larger variant)

ps:  I'm aware that charging from a computer USB port can be much slower than using the wallwart, because the computer USB ports can sometimes be current-limited to 500 mA.
Lies, damned lies, and equipment specs.  I can print up a lovely sticker that says it'll output 1000A.  Doesn't mean it'll output that.

It's significantly cheaper to make a low-power supply than it is to make a high-power power supply.  Making the latter means you need a bigger transformer (with heavier windings), larger capacitor(s), larger inductor(s) and higher current rectifier(s), all of which add up to more money.  If you're trying to make stuff on the cheap and aren't worried about pesky things like ethics, you'll go with the lower end components and massage the specs (by testing at low temperatures, etc.  This is especially common with computer power supplies which specify output at 25C, even though they would actually operate at ~40C unless you're using them inside a refrigerator), use theoretical values rather than actually testing the product (saving money on QA), or just outright lie about the specs, the latter of which is probably the case with this charger, for reasons below.

One big red warning light that your charger is substandard is that the CE mark (which is used to indicate conformance to EU standards) is fake.  The shape of the letters and their spacing is wrong.  A real CE mark should have the letters each forming a half-and-a-bit circle and should link up if you continue the arc, as shown below.  A cursory glance at the mark on your charger shows it looks nothing like this.



The CCC mark (indicating compliance to Chinese standards) is also fake.

Other warnings signs include having no manufacturer listed and improper capitalization (should be mA, not MA, unless they're claiming is can supply a billion amps.).

In essence, to determine if a charger is likely to be substandard, apply the same thoughts as you would for determining whether a product is counterfeit.  The only real difference between a substandard product and most counterfeits is whether a manufacturer's label is forged or not.