Use the g_scratchpad to pass information to the client side, when the form loads, to reduce the load time.

//*** Client script ***
if(g_scratchpad.hasAttachments && g_scratchpad.instance == "production") {
	var comment = "Something is attached to a child of " + parentCaller + "'s record.";
	g_form.setValue('comments', comment);
}

An onDisplay business rule to prepare all the data for client side on g_scratchpad

//*** onDisplay business rule ***
g_scratchpad.hasAttachments = current.hasAttachments(); //Does this record have attachments?
g_scratchpad.instance = gs.getProperty('instance_name'); //Get the name of the instance
g_scratchpad.parentCaller = current.parent.caller_id.name; //Get parent's caller's name

Call the asynchronous script include from the client script, and also receive the answer back from the server after processing

function onChange(control, oldValue, newValue, isLoading) {
	if (isLoading) {
		return;
	}
 
	var ga = new GlideAjax('serverCheck');
	ga.addParam('sysparm_name', 'serverFunctionName'); //sysparm_name is reserved for the name of the script include function to call
	ga.addParam('sysparm_group', g_form.getValue('assignment_group'));
	ga.addParam('sysparm_other_variable', g_form.getValue('short_description')); //Any variable can be sent
	ga.getXML(clientCallback); //Use getXML rather than getXMLWait. Input is your callback function
}
 
function clientCallback(response) {
	var answer = response.responseXML.documentElement.getAttribute("answer"); //Get answer from server
	//Do something with answer
	g_form.setValue('short_description', answer);
}

Asynchronous GlideAjax script include, called from the client script with the script include name and the sysparm_name of the function.

var serverCheck = Class.create();
serverCheck.prototype = Object.extendsObject(AbstractAjaxProcessor, {
	serverFunctionName: function() {
		var assignment_group = this.getParameter('sysparm_group');
		var short_desc = this.getParameter('sysparm_other_variable');
		var returnValue = "";
 
		var groups = new GlideRecord('sys_user_group');	
		if(groups.get(assignment_group)) {
			returnValue = groups.name + ": " + short_desc;
		}
		return returnValue;
	}
});

When setting a reference value, remember to include the display value, otherwise ServiceNow will make another call to the server

//Reference field
var group_name = 'IT Support';
var group_id = '86e8d203ff833100ba13ffffffffff70';
g_form.setValue('assignment_group', group_id, group_name); //No server call required
 
//Other fields
var short_desc = "This is my new short description";
g_form.setValue('short_description', short_desc);