When you order a Raspberry Pi 3 then do yourself a favour and also order the matching 5.1VDC 2.5A power supply (eg: STONTRONICS T5875DV, Element 14 item 2520785). The RPi3 is four cores of 64-bit ARM with an impressive GPU -- that's a lot to power. If you present it with too little power the circuitry will make the red "power" LED blink and the software will reduce the CPU's clock rate.
You'll notice the clever use of tolerances to allow the RPi3 power supply to charge a phone, as you might expect from its Micro USB connector (5.0V + 10% = 5.5V, 5.1V + 5% ≅ 5.4V). The cable on the RPi3 power supply has an impressive amount of copper, so they are serious about avoid voltage drop due to thin cables.
You can argue that this is poor design, that the RPi should really use one of the higher power delivery solutions designed for mobile phones. But with Google, Apple and Samsung all choosing different solutions? Whatever the RPi's designers chose to do then most purchasers would have to buy the matching power supply. At least this design is simple for makers and hobbyists to power the RPi3 (simply provide the specified voltage and current, no USB signalling is needed).
The RPi3 will also slow down when it gets too hot; this is called throttling and is a feature of all modern CPUs. People are currently experimenting with heat sinks. Even a traditional aluminium 10mm heat sink seems to make a worthwhile difference in preventing throttling on CPU-intensive tasks; although how often such tasks occur in practice is another question. The newer ceramic heat sinks are about four times more effective than the classic black aluminium heat sinks, so keep your eyes out for someone offering a kit of those for the RPi3. This is a further complication when looking at cases, as the airflow through most RPi2 cases is quite poor. I've simply taken a drill to the plastic RPi2 case I am using, although there are ugly industrial cases and expensive attractive cases with good airflow.