jQuery ajax等待时间长

Question

I'm using jQuery ajax to call a webmethod in my asp.net page, here's the code:

$.ajax({
            type: 'POST',
            url: 'Home.aspx/GetPlantInfo',
            contentType: 'application/json; charset=utf-8',
            dataType: 'json',
            success: function (response) {
                info= JSON.parse(response.d);
                $("#PlantName").val(info.Name);
                .
                .
                .
            },
            failure: function (r) {
                alert(r.d);
            }
        });

the webmethod does a simple linq query and serializes the result to a JSON string, which takes less than half a sec. here's the webmethod code:

        [WebMethod]
    public static string GetPlantInfo()
    {
        MyDataClassesDataContext dc = new  MyDataClassesDataContext();
        Plant p = (from x in dc.Plants
                       where x.Name == "someName"
                       select x).FirstOrDefault();
        string plantJson = JsonConvert.SerializeObject(p);
        return plantJson;
    }

however the ajax call takes 10-15 seconds to complete. I inspected ajax timing and apparently most of this time is "Waiting" time, with negligible send and receive time. I googled this problem with no lock. What I'm asking is what exactly is this "Waiting" time? Is the problem from IIS server? is something wrong with my code?