If you install all of the extensions to ram, all programs are loaded to ram when you start the computer. When you start a program, the computer will access it from ram. If you install a lot of programs, and use this method, you may require more ram than most other operating systems.
If you mount extensions, the operating system knows where they are on the hard drive. When you start a program, the computer will access it from the hard drive. This method only requires the same amount of ram as other operating systems.