ADC is slow

I'm hard-pressed to find any documentation on what the ADC is actually capable of in terms of sampling rate. Currently, the staging release can only do about 2.5kHz. I've found others using the ADC at 20kHz and higher successfully, but not with this toolchain.

For example: http://www.esp8266.com/viewtopic.php?f=13&t=2134

and

https://hackaday.io/project/4318-vu-meter-esp8266-ws2812b

Any chance on improved ADC sample time?
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