$('.navbar-nav .nav-item.dropdown').hover(
  function() {
    var $dropdown = $(this);
    clearTimeout($dropdown.data('timer'));
    $dropdown.data('timer', setTimeout(function() {
      $dropdown.addClass('show');
      $dropdown.find('.dropdown-menu').addClass('show');
    }, 250));
  },
  function() {
    var $dropdown = $(this);
    clearTimeout($dropdown.data('timer'));
    $dropdown.data('timer', setTimeout(function() {
      $dropdown.removeClass('show');
      $dropdown.find('.dropdown-menu').removeClass('show');
    }, 400));
  }
);