Your content function should be a bit more complicated, the possible solution is:
content: function (callback)
{
var $this = $(this), isMouseOn = true;
// check if tooltip ajax-request is in progress
// really needed for slow scripts only
if($this.data('tt-busy')) return;
// check if el is hovered (in jquery <= 1.8 you can simly use .is(':hover')
$this
.data('tt-busy', true)
.on('mouseenter.xxx', function() { isMouseOn = true; })
.on('mouseleave.xxx', function() { isMouseOn = false; });
//
$.get('slow.script', function(d) {
if(isMouseOn) callback(d);
}).always(function() {
// even if result fails set loading var to false and unbind hover events
$this
.data('tt-busy', false)
.off('.xxx');
});
}