Required to enable Admin theme

In my customer site ThemeKey is used to apply the administation theme (Seven) when user has the admin or editor role. I noticed that applying the admin theme with ThemeKey requires the Seven theme to be enabled. But this is bad for performance. The boot time increases measurably when the Seven theme (or any additional theme) is enabled. I'd rather not. When selecting the 'Administation theme' is it required to enable it? Technically this is not. I used hook_custom_theme() and it works with a disabled theme.