Delay in receiving Ajax results using jQuery

I have a simple jQuery Ajax call which gets HTML results from a PHP file (which just executes a mysql query and formats the results in a list). The problem I'm having though is that this whole Ajax call takes about 12s to execute (the 'Waiting for Response' time in Firebug is 12.3s), but when I just load the PHP page wit开发者_JAVA百科h the same URL parameters in my browser, the whole page is loaded in about 300ms (and when I just run the SQL query that the PHP file calls, it takes under 1ms).

Some other notes which may help:

• The size of the Ajax data is 2kb, so I don't think its due to that.
• The data takes much quicker to load (about 4.3s) if the same ajax call is done with the same parameters after its been run once before already (even though I have the Cache-Control for the page set to: private, no-store, no-cache, must-revalidate, post-check=0, pre-check=0)

Here is the jQuery source code for the ajax call:

$.ajax({
            url:    "./lib/hippofunctions.php",
            type:   "GET", 
           dataType: "html",
            data:
                {
                    "ajax_action" : "get_hippos_for_tags",
                    "curTags" : currentCheckedTags,
                    "sortBy" : sortBy, 
                    "params" : checkedFriendsStr,
                    "location" : location,
                    "start"  : start
                },
            error:  function(request){my_alert("error on ajax_getHipposfortags ")},
            success:function(data){ 
                $("#HippoContainer_inner").html(data);
        }
        })
    }

any thoughts or suggestions why its taking so long to receive the Ajax data and how I can speed this up?

I'll update this with more suggestions, but for now do some performance testing on the success function:

success:function(data){
 console.time("data render");
 $("#HippoContainer_inner").html(data);
 console.timeEnd("data render");
}