Seems to be related to the html() call that actually shows the content really fast, then hides it again before the fadeIn kicks in. If the content is not updated fadeIn/out behaves as expected.
You can use a .hide() in the success function:
beforeSend: function () {
$("#myDiv").fadeOut("slow").empty();
},
success: function (data) {
$('#myDiv').hide().html("test!").fadeIn("slow");
},