None of the four main desktop options are particularly noted for being quick at the login or to start applications. If speed is the most important criterion for your choice of desktops, you are probably better off with Xfce, LXDE or others that have enjoyed their own small upsurges in popularity in recent years.
Unity’s simple appearance leads some users to assume that it uses less memory than the other leading desktops. However, running the command free -m indicates that, while all four major choices will seize as much of the first couple of gigabytes of RAM as they can, Unity consistently uses both more RAM and more buffers than the other three. Even GNOME 3 running on Ubuntu uses considerably less RAM and fewer buffers.
By contrast, KDE 4 has a reputation of being bloated. Perhaps early in the current release series, the reputation was justified. However, KDE has been rewriting much of its interface functionality in recent releases, and the 4.10 release uses about 55 percnet of the memory that Linux Mint’s Cinnamon/Mate does, and about the same number of buffers. These figures support what my subjective eye tells me: KDE and Cinnamon/mate are the fastest of the four main choices.