Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
If you do a lot of work on a Linux computer, continuously switching between many windows, the right window manager can make you much faster and more productive than an extra 2GB of RAM.
In this context, 'right' means any combination of two different qualities: raw speed and correspondence with your actual needs, habits and personality. If you need to make the most of an obsolete PC, you'll probably want something slimmer and nimbler than either Gnome or KDE.
In other cases, what saves you more time is whatever does by default, with one click or keystroke, what you want to do most often. Be it vertical maximization, iconization or jumping around virtual desktops. The possibility to use the mouse as little as possible is another big productivity boost.