Faster Uploading from ESP8266

I am using NODEMCU 1.0(ESP12E) with Arduino 1.6.12. When I connect A0 pin of NODEMCU to 3.3V of the same board. I get analog reading as 983. Why I am not getting 1023.
If I map it from 0-1023, the accuracy won't increase

I have no idea, but you might get more responses if you change your topic title (currently "Faster Uploading from ESP8266") to something that matches your question (such as "ESP8266 ADC maximum value is only 983...not 1023").

PS: might be a hardware issue