Set How Much Memory is Allocated to Windows
Parallels Desktop is set to allocate a certain amount of your Mac's available RAM memory to Windows and the rest to Mac OS X. If too much memory is allocated to Windows, Mac OS X may slow down, which in turn can cause Windows programs to slow down because they're running on top of Mac OS X. If, on the other hand, too much memory is allocated to Mac OS X, then Mac OS X applications may run well but Windows programs might slow down.
To optimize the performance of both Windows and Mac OS X, set the memory allocated to Windows to be somewhere within the recommended range.
To set how much memory is allocated to Windows:
If you change this setting and then you don't like how Windows is performing, then try a different setting.
Note: If you want to prevent Virtual Machine Configuration from unauthorized changes, click the Lock icon at the bottom of the window. The next time someone wants to change the settings in any pane of Virtual Machine Configuration, an administrator's password will be required.