Folks, keep in mind that cable quality will vary a great deal, too. I have had two like cables from different suppliers charge at significantly different rates using the same charger and same device.
Generally speaking, USB 2.0 and 3.0 cables of like electrical quality should charge the device at the same rate from the same power supply. The USB3 half of the connection only sports enhanced data lines, using a separate set of wire pairs in the 3.0 side of the cable that meet the 3.0 spec. A USB3 cable on a USB2 data port will only use the "USB2" side of the device connector. Likewise, the power pair only runs through the "USB2" side, for both USB3 and USB2 connections.
All of this is in the context of a device connected to a charger, not a computer USB port. The rules about charging differ when connected to a data source like a computer or tablet.
So far, writing in mid-2014, USB3 cables typically follow the official spec for wire guage (read "size" or "diameter") for power and data wires. As more bargain manufacturers take up the market then USB3 cables, just like USB2 cables, will become available at very cheap prices for cables that do not follow the spec, leading to slow charging and sub-optimal data performance. Thinner wires lower the price and decrease the charging rate (smaller wires lose more power to resistance than bigger wires). All wires present resistance, but not all wires present the same resistance.
Two things increase the resistance:
- Like thin garden hoses passing less water, thinner wires simply carry fewer electrons over a given distance, and lose it more quickly as the distance increases. This is the downfall of cheap cables.
- Longer wires, simply because a given number of electrons have to travel a longer distance, lose more power the further they go. It is not linear...like "twice the distance being half the charge", but the further it must travel the faster it will drop (exponential). This is where our desire for long cables bites back.
Some chargers, also sometimes called "power blocks", include circuits that identify the connection to the device and adjust their output accordingly. Some do so to "tune the rate of charge" (efficiency), and others do so to "prefer my brand devices and charge them faster" (marketing by negative experience).
Samsung chargers for the most part seem to hold to the basic "charge as best you can", making the full output amperage available, leaving it up to the phone to draw what it can for charging. The cable again plays into the equation, but Samsung USB2 and USB3 cables seem to be consistently good compared to bargain cables.
My best charging comes from either my Samsung USB3 standard cable or my decent 18-inch/46-cm after-market (short) USB3 cable. After that comes a stock Samsung USB2 cable from an older Galaxy S2...it's pretty thin so I imagine the power pair is smaller than the power pair in either USB3 cable. On my Samsung 2.1a charger with the USB3 cable I can get my powered-down Note 3 to 100% in somewhere around 90 minutes. The thinner Samsung USB2 cable does the same in not quite 2 hours.
Chargers are a whole other world of possibilities. Their performance will generally decrease over time (on the order of years), especially if there are other noisy electrical devices on the same electrical circuit. Air conditioners, refrigerators, heaters, other things with larger motors like fans (if they use cheap motors), and so forth, create additional strain that both shorten the working life and impair charging performance at any time. Your line power matters, too...some areas see lower average line voltage than others, usually because of older in-home wiring or "end of a long leg" service usually seen in rural areas, as well as areas where the electrical service in the area has been pushed to its limits (high-growth in short-time).
Chargers go bad, too. Does not matter that they don't have moving parts...the kind of work they do creates heat that ages the parts, including the included electronics in the charger. Low line power and high line power (too little and too much) adds to the heating problem. On top of that, some percentage of new blocks fail in the first 100 hours of service (weeks to months) while the blocks that followed them down the line last years...luck of the draw. Samsung's quality assurance processes seem to weed those out of their production line...I've yet to have one of their chargers die on me, where I have had a number of cheap chargers go bad.
I tell folks to buy name-brand chargers, whether Apple or Samsung or Asus, and look for cables that spec the wire gauge in AWG, usually 28/24...that's "data/power", sometimes reversed to 24/28 for power/data...the smaller number (24) is a thicker wire, which is better for charging at a higher amperage. Also, go for the shortest cable you need, in order to to reduce resistance and improve charging even more. A 1m cable will charge more slowly than a 1/2m cable of the same quality, simply because the electricity has to travel through more (longer) wire.
All of this is from my many years experience working with all sorts of things that use chargers of one sort or another, and more recently chargers that accept any USB cable. I have not measured these on any meters (yet) or other scoping equipment, so my answers are based on experiential evidence and not on actual testing with tools. The principles are valid, though some facts may vary (Samsung may have lots of charger smarts that twiddle their own "knobs", or may not). IOW, most often right, sometimes slightly wrong, rarely dead wrong.