The source of an autocomplete of jQuery UI is requested every time I press a key

Why is the autocomplete executed every time I press a key?

This is the code used:

$('#xmarcaId').autocomplete({    
     source: "productosServices17.html?opcion=PRUEBA",
     minLength:1,
     select: function(event, ui){

         // just in case you want to see the ID
         var accountVal = ui.item.value;
         console.log(accountVal);

         // now set the label in the textbox
         var accountText = ui.item.label;
         $('#xmarcaId').val(accountText);

         return false;
     },
     focus: function( event, ui ) {
         // this is to prevent showing an ID in the textbox instead of name
         // when the user tries to select using the up/down arrow of his keyboard
         $( "#xmarcaId" ).val( ui.item.label );
         return false;
     }

});