There is a small improvement that can be made to to the program we wrote last time. It involves taking advantage of the last parameter to the IContextMenu::QueryContextMenu method:

This flag is set when the user is activating the default action, typically by double-clicking. This flag provides a hint for the shortcut menu extension to add nothing if it does not modify the default item in the menu. A shortcut menu extension or drag-and-drop handler should not add any menu items if this value is specified. A namespace extension should add only the default item (if any).

As the text from MSDN indicates, this flag is a hint to the IContextMenu implementation that it should worry only about the default command.

void OnContextMenu(HWND hwnd, HWND hwndContext, UINT xPos, UINT yPos)
  IContextMenu *pcm;
  if (SUCCEEDED(GetUIObjectOfFile(hwnd, L"C:\\Windows\\clock.avi",
                   IID_IContextMenu, (void**)&pcm))) {
    HMENU hmenu = CreatePopupMenu();
    if (hmenu) {
      if (SUCCEEDED(pcm->QueryContextMenu(hmenu, 0,
                             SCRATCH_QCM_FIRST, SCRATCH_QCM_LAST,
                             CMF_DEFAULTONLY))) {
        UINT id = GetMenuDefaultItem(hmenu, FALSE, 0);
        if (id != (UINT)-1) {
          CMINVOKECOMMANDINFO info = { 0 };
          info.cbSize = sizeof(info);
          info.hwnd = hwnd;
          info.lpVerb = MAKEINTRESOURCEA(id - SCRATCH_QCM_FIRST);

With this change on my machine, the time taken by the call to IContextMenu::QueryContextMenu dropped from 100ms to 50ms. Your mileage may vary. It depends on how many context menu extensions you have and how well they respect the CMF_DEFAULTONLY flag.

(And this exercise highlights how important it is that people who implement the IContextMenu interface pay attention to the flags. If your context menu handler doesn't respect the CMF_DEFAULTONLY flag, then you're part of the problem.)