Why USB wall chargers don't reach their amperage rating?
Hooking up a simple resistor to modern chargers is not a good idea to gauge performance. Most modern chargers are "smart" chargers. They need some sort of "handshake" from the device to determine what current limit to enforce. A simple resistor will likely result in the lowest current limit being applied. This concurs with your results: the cheapest charger is likely ignoring the signaling standards and gives you the highest current limit anyways.

Here is a list lifted from apcmag. It tells you the voltage that must be present on the D+ and D- lines respectively to set a current limit: 


2.0V/2.0V – low power (500mA)
2.0V/2.7V – Apple iPhone (1000mA/5-watt)
2.7V/2.0V – Apple iPad (2100mA/10-watt)
2.7V/2.7V – 12-watt (2400mA, possibly used by Blackberry) D+/D- shorted together – USB-IF BC 1.2 standard
1.2V/1.2V – Samsung devices


Keeping in mind this list, it might therefore be your power bank not applying the right signal to the charger.

Furthermore, what cable are you using? Cheap and long USB cables from China can have relatively high resistance (even a few ohms!). So you might be dropping voltage there instead of from the charger itself. 

Furthermore, many cheap charge banks indeed charge very slowly by design. So please define "slow". What is the rated capacity of your charge bank? Seems like a charge bank problem rather than a charger problem. Battery capacity (mAh)/rated charge current(mA) = best case charge time(hours). Is this expected?

EDIT 1: The use of the word handshake (in double quotes for a reason) is perhaps misleading. The "handshake", here, simply consists of the voltages given in the table above, applied across the the D+/D- lines, if this isn't clear.