The more power a USB outlet provides, the faster it charges your phone. A new Android app named 'Ampere' answers the question. I installed it on my new UMI Super cell phone, and then went around plugging it into a variety of USB outlets. This phone uses the new USB-C port, and supports fast charging.
With stories of mini-chargers bursting into flame, I've been on a quest to replace all stand-alone chargers in our home. I now use UPSes [uninterruptible power supplies] that include USB ports, power bars from reputable makers that include USB ports, and replaced three wall outlets with ones that have USB ports.
TIP: Check that the devices claim to support at 3.5A output for the two USB ports that are usually provided. You need 2.5A to charge most Android tablets, leaving 1.0A to simultaneously charge a phone.
Here are my results. Green indicates good results (high power output), red = poor results.
Official UMI charger -- 1940mA
All wall outlets with USB ports -- 1860mA
One UPS -- 1940mA
Another UPS -- 1870mA
One power bar -- 1930mA
Another power bar -- 490mA
Computers -- 460mA - 480mA
I was disappointed to learn that new computers with USB 3 ports provided no more power than older ones with USB 2 ports.
I suppose the real test is with portable power supplies, the ones we carry with us to recharge phones while on-the-go. I tested three that I have. Two larger ones (that I bought) output at 1940mA, while a smaller one (that I got free from a vendor) was just 490mA.
If you use Ampere, note that you may have to adjust two settings to get a power reading: in Basic Settings, you may have to toggle "Old Measurement Method" as well as change the Measurement Interface.
A caveat. I don't know if this software measures the current flow correctly, but the numbers seem reasonable. My UMI Super phone got the Android 7.01 upgrade over the weekend, which fixed a problem with fast charging. Prior to this update, the highest numbers were around 1500mA -- so a 450mA improvement. In other words, the phone charges 30% faster on more powerful chargers with the OS update.