It is due to your "my.js" configuration file. You need to change the
last line in the file from "default.js" to "my.js" so that MathJax
will be informed that your file has completed loading (right now you
are telling MathJax that default.js has loaded, which is the wrong
file). MathJax has a 15 second timeout waiting for files to indicate
that they have loaded properly, so that is the 15 second delay you are
seeing.
Davide