Ajax function with a callback I have Ajax function with a callback which fetches all the Patient data however I have some perfomance issues and trying to figure out what it might be, does anyone have idea? That's my Code

var patient = new Array();
function sync(arg, callback){   //ajax result 
    $('.loader').show();
    $.ajax({ 
        method: 'GET',
        url: 'sync/active.php',
        dataType: 'json',
        data: arg, // argument schicken und aktualisieren
        success: function(data, status, xhr){
         $('.loader').hide();    
         callback(data);
        // setTimeout(sync, ajaxDelay);
        },
        error: function(xhr, ajaxOptions, thrownError){
            console.log(thrownError);
        }
   });   
}
function onPatientCallback(data) {
        var res = data; 
        for(var i=0; i&lt;res.length;i++){
            for(var key in res[i]){
                var value = res[i][key];
                switch(key){
                    case "id":
                        res[i][key] = parseInt(value);
                        break;
                    case "kundennr":
                        res[i][key] = parseInt(value);  
                        break;
                    case "client":
                        res[i][key] = value;
                        break;
                    case "start":
                        res[i][key] = new Date(value);
                        break;
                    case "end":
                        res[i][key] = new Date(value);
                        break;
                    case "title":
                        res[i][key] = value;
                        break;
                    case "description":
                        res[i][key] = value;
                        break;

                    case "termart":
                        res[i][key] = parseInt(value);
                        break;

                    case "userId":
                        res[i][key] = parseInt(value);
                        break;

                    default:
                        console.log("unbekannter Datentyp "+key);
                }
            }
        }
        patient = res;

}


I use the function to fill the patient variable and call it in another js file like that  sync({calling: "patient"}, onPatientCallback);
From a short review;


patient is a global variable, global variables are bad
var patient = [] is more idiomatic than var patient = new Array();
Comments should be all German or all English (I would go for all English, its the common language of the developers)
You can group switch labels, this is valid JavaScript:

        switch(key){
            case "id":
            case "kundennr":
            case "termart":
            case "userId":
                res[i][key] = parseInt(value);
                break;
            case "client":
            case "title":
            case "description":
                res[i][key] = value;
                break;
            case "start":
            case "end":
                res[i][key] = new Date(value);
                break;
            default:
                console.log("unbekannter Datentyp " + key);

Your data fields should be either all English or all German (you have userId, but also kundennr), here as well I would go for all English
Since value already contains res[i][key] for "client", "title", etc., you should probably comment on that 
Since this is real, you should not log to console for an unknown data type, you should call a REST service that will log this entry in a database table, and even possibly send out an email to developers 
var key in Object.keys(res[i]) is safer than var key in res[i], you never know when someone decides to extend Object
Your indenting is off, consider using a beautifier