Interesting. I'd expect that to only cause a slow compilation on the first compile, then faster compilations on subsequent compiles until you change the board or edit a core file. I'll have to do some tests on my machine to see whether that setting is actually helpful. Well I'm glad you found a solution. That must be quite a relief! Compiling for AVR boards is reasonably fast on my computer but compiling for the ESP8266 is painfully slow and that makes the development process really drag. I end up trying to multitask while waiting for the compilation to finish and then getting distracted and losing track of exactly what I was doing in my programming project.