What is fast charging

Except QC2.0/QC3.0 is otherwise noted, the term "fast charging" or "quick charging" at present means it can charge your device at the possible fastest speed your device need rather than the charger can max put out.
Every device has its unique charging protocol. Charging output can not solely determine charging speed. For example, your phone need 1A charging current, then there is no difference between a charger of 1A output and a charger of 2A output. You phone can only take 1A. If your device need 2A, then obvisiouly 2A charger exceeds in charging speed.
Back to the topic, fast charger can test your device' protocol and intelligently adpots the best compatiable charging current, which is friendly to your device and best for the speed.
As for the cable, different cables have different inner resistance which influence the charging speed. Cables from certificated thrid parties are always recommended.
Hope it helps!

Because 9v @ 1.67a is more output than 5v @ 2a.

And the Quick Charge 2.0 and 3.0 doesn't do the fast charging at 5v. I have a USB multimeter tool, and have tested/ observed QC 2.0 "fast charging" my S7 Active.

So, in short, you'll need some kind of charger that supports the QC 2.0 or 3.0 standard, and a good cable, to see what the fast charging is all about.

As others here have mentioned, fast charging is really what the limits of your device is able to accept.

If you have smartphone that is able to accept a 1.0A of charging current and you have a Wall charger that has a max Output of 2.4A per port, then the smartphone will only charge at 1.0A. Not 2.4A, because the phone would be fried.

With that said, it's best to aim for chargers that are in the 5V/2.0A or 5V/2.4A charging Output category. This ensures that whatever device you connect to the Wall charger, Car charger or power bank will charge as fast as possible.

5V/2.4 charging speed is really meant for tablets like a iPad because they can accept such a large Output current.

Quick Charge 2.0 and 3.0 is compatible with whatever smartphones have Qualcomm's Snapdragon compatible processors. There are a lot of Quick Charge 2.0 compatible devices but not so much for QC 3.0, because it's somewhat new. However, QC 2.0 can still work with QC 3.0 chargers but they'll just default to a QC 2.0 charging speed.

Quick Charge 3.0 device can work with a QC 2.0 charger and will charge at Quick Charge 2.0 charging speed. Quick Charge 2.0 is the charging technology that plunged charging speeds into the 3 Amp category. Then there's Quick Charge 3.0 that actually doesn't charge any higher than 3 Amps but it does provide a higher Voltage than Quick Charge 2.0, making the charging process more efficient.

So in the order of what "Fast Potential Charging" is:

• 5V/1.0A
• 5V/2.0A
• 5V/2.1A
• 5V/2.4A
• Quick Charge 2.0 (Compatible with Quick Charge 2.0 & 3.0 devices)
• Quick Charge 3.0 (QC 2.0 will charge at QC 2.0 charging speed)

To learn more about Quick Charge tech in detail and better understanding of it check it out here. If you're a bit lost on the terminology or want to understand better what charging charging tech entails, then you can learn some right here.