By setting $merge_compiled_includes to TRUE Smarty will merge the compiled template 
       code of subtemplates into the compiled code of the main template. This increases rendering speed of templates 
       using a many different sub-templates.
      
       Individual sub-templates can be merged by setting the inline option flag within
       the {include} tag. $merge_compiled_includes does not have to
       be enabled for the inline merge.
      
<?php
$smarty->merge_compiled_includes = true;
?>
        
This is a compile time option. If you change the setting you must make sure that the templates get recompiled.
      See also
        {include} tag