Thread by @ErrataRob

1/ Let's talk USB power for a moment. Even if you buy a good USB power supply and cable, you still may not be able to run a Raspberry Pi at full 1.4-GHz without it throttling back to 600-MHz because off low voltage issues.

2/ I'm not talking about current limits, which is another problem. I'm assuming you already have a power supply that can provide 2.4amps. Instead, I'm talking about Ohm's Law.

3/ There are two equations you care about:
P = V*I (power/watts = voltage times current/amps)
and
V = I*R (voltage drop = current/amps times resistance)

4/ The first equation means that in order to get power from USB's 5 volt standard, you need four times the current as for other standards that use 20 volts.
10 watts = 5 volts times 2 amps
10 watts = 20 volts times 0.5 amps

5/ This makes USB is horrible standard for power delivery. It wasn't designed for power. The original standard called for a maximum of 0.5 amps supply to devices plugged directly into the port with at most a short cable.

6/ Resistance in a cable is proportional to length, so doubling the length of the cable doubles the resistance. Doubling the resistance doubles the voltage drop. Instead of 6 inch cables we now have 6 ft cables, which is 12 times the voltage drop.

7/ Things work fine when voltage drops from 5.0 volts to 4.9 volts, it's within tolerances of the spec. Using a 3 ft cable instead of 1 ft cable triples the voltage drop, so now 4.7 volts, which is outside the spec, causing the Raspberry Pi to throttle.

8/ Thicker wires mean less resistance and less voltage drop. The thickest power cables are 20 AWG, the thinnest are 28 AWG (bigger numbers mean thinner wires). There is a 5-to-1 difference in resistance, and hence voltage drop.

9/ But even high quality 3ft/1m cables using thick wires I get from Amazon cause enough voltage drop that the Raspberry Pi throttles. But there is a solution: most power supplies boost the voltage.

10/ The "official" Raspberry Pi power supply I got for $10 boosts the voltage to 5.25 volts, so when I run the maximum current through it (2.4 amps), the voltage drops down to 4.91, across it's 3 foot cable.

11/ Modern cell phone "chargers" do another trick. They typically start at 5.1 volts, but they have an added chip that tries to measure the length of the cable. When they detect long 10 foot cables, they'll boost the voltage past 5.5 volts.

12/ I have a "Volutz" 10 foot cable using thick 20 AWG wiring. At 2.4 amps, it still delivers 4.74 volts on one end with the charging power supply pumping in over 5.5 volts at the other end, thus handling the almost 0.6 volt drop.

13/ So the solution to your Raspberry Pi power problems, if you aren't buying the "recommended" power supply, is a power supply that boosts voltage, thicker cables, and shorter cables.

14/ I tested a bunch of cables on Amazon.com. Vendors are somewhat deceptive. A good solution is to buy "Anker" products. They are very popular, and provide high quality products for a low price.

15/ Their multi port charging stations boost the voltage. Their cables have thick wires. I've tested then with a USB load generator and power meter. The "Voluttz" cables are also worth mentioning.

16/ I hooked up a bunch of cables to an Anker 6 port charger, which boosts voltage, and I got a wide range of results. There are a lot of sellers of "charger" cables with "wide" wires (e.g. "Agvee") that sell crap, though still better than some data-only (non-charging) cables.

17/ These tests were at 2.4 amps. The Raspberry Pi B 3+ draws around 0.4 amps when idle, and 1.3 amps while at 100% CPU load.

18/ Sadly, chargers don't advertise whether they boost voltage for wire length resistance, so it's hard finding a good charger. But most that advertise themselves as 2.4 amp (or above) seem to have this feature anyway.