Data acquisition with Python on a Beaglebone Black - Increasing sample speed
The library you are using is not capable of doing what you want.  It can only deliver single values as fast as your loop can read them.



You need a different library that can access the hardware better.   

The beaglebone black has what is called a programmable real-time unit (PRU.) These can be programmed to do things that need to be done quickly - like sampling the ADC at high speed.

PyPRUSS gives access to all the PRU functions with a Python API.  It may be overkill for what you need.

There's also beaglebone_pru_adc which wraps just access to the PRU ADC  functions.  It can deliver single values on request, but it also has what the developer calls "oscilloscope" mode, in which it takes a bunch of samples and delivers the them all back in one packet. 

One or the other of those two ought to do the trick.