$Smarty = new Smarty;
$Smarty->registerFilter(Smarty::FILTER_POST, 'lambdavars');
$Smarty->assign(array(
// Var is always filled, even if it's not used in template
'SIMPLE_VAR' => loadBigData(),
// Var is filled only when it's really used in template
'LAMBDA_VAR' => function() { return loadBigData(); },
));