The POSIX and OS2 subsystems don't take any resources until an application that uses them is started.
If you open %SystemRoot%\System32\Config.NT, you will see the following comment:
REM DOSONLY REM By default, you can start any type of application when running REM COMMAND.COM. If you start an application other than an MS-DOS-based REM application, any running TSR may be disrupted. To ensure that only REM MS-DOS-based applications can be started, add the command dosonly to REM CONFIG.NT or other startup file.You can disable these optional subsystems by using Regedt32 to navigate to:
Select the Posix and/or Os2 value names and delete them.