It could be that the read/write file does time wise not fit into the window the ESP8266's wifi stack provides for other code to run before it has to answer its next own interrupt. An ESP8266 has just not much cycles left to do other (demanding) things. To stay (far or absolutely) away from this issue, go for a two processor setup: an Espruino board and just for wifi pair it with any ESP8266... that lets you skip a lot of issues and saves you tons of time... way beyond the money you spend. Reliable (1 'wire', and radio/RF IS one wire) communication requires dedicated resources, because it is all time sensitive serialized...
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
It could be that the read/write file does time wise not fit into the window the ESP8266's wifi stack provides for other code to run before it has to answer its next own interrupt. An ESP8266 has just not much cycles left to do other (demanding) things. To stay (far or absolutely) away from this issue, go for a two processor setup: an Espruino board and just for wifi pair it with any ESP8266... that lets you skip a lot of issues and saves you tons of time... way beyond the money you spend. Reliable (1 'wire', and radio/RF IS one wire) communication requires dedicated resources, because it is all time sensitive serialized...