In EE 2.9+ there is a new parser included. The neccessity for this extension is somewhat diminished. the if logged_in_member_id conditional won't execute tags anymore if the condition is not met. Test your situation first with the template debugger.

I never understood why (if logged_in) is parsed so late in the template. If you have a channel:entries tag for members, and another for visitors, both get rendered in the template, and one of them is discarded when advance conditionals are parsed. (see your template debugger).

This extension adds logged_in_member_id and logged_in_group_id to the global variables array so they are parsed early and can be used in simple conditionals. Try the following code with the extension disabled and enabled and see the difference in the template debugger output.

{if logged_in_member_id != "0"}
  {exp:channel:entries status="open|members_only"}
  ...
  {/exp:channel:entries}
{/if}
{if logged_in_member_id == "0"}
  {exp:channel:entries status="open"}
  ...
  {/exp:channel:entries}
{/if}

If you rely, for some reason, on logged_in_member_id being parsed late, this extension can also add global_member_id and global_group_id for convenience.