Look at this : https://github.com/esp8266/Arduino/issues/2121
Summary : Don't put a lot of css, js files. Merge them to a single css and single js if possible or maximum 2 css and 2 js files. Don't zip font files. Some of local js files need other files like *.map or something, becareful don't forget to put them in same folder.
Main problem is count of files to read thats all.