Assumeing you mean MVC -> ASP.NET MVC or something HTML/Javascript based with some sort of web server component.
In general you would have an animated gif (A spinning wheel for example) and show hide them while you are waiting for a long running operation.
In Javascript you can take advantage of Promises or use generic callback functions.
//pseudo code
loadData(function(data){
// data came back async
// do something with data
$('#loader').hide();
});
$('#loader').show();
or with promises
//method should return promise
var promise = loadData();
$('#loader').show();
promise.done(function(data){
// do something with data
$('#loader').hide();
});
you should of course do handle the error case too but same principles apply....