Ubuntu originally has several available desktop environments to be chosen during login (like Gnome and Unity). I want to install others like KDE, LXDE, XFCE, etc.
I wonder if installing many other desktop environments/user interfaces will slow down the system? Or they are static files reside on the disk, and no effect on the system when their corresponding desktop environment are not used?
My compliments for the desire to try out alternatives, and not stick to 'out of the box' -- that's the FOSS attitude!
Unless you have concurrent users running different desktops at the same time, the other desktop environments will not slow down the system. They are mainly sitting statically on the disk.
In fact, you are likely to find out that some lightweight desktop environments will provide a snappier user experience overall, notably on lower end machines. If you particularly like one, you could even consider replacing the ubuntu-desktop package by the desktop environment of your choice.
If you want blazingly fast (and completely austere), try running only X with a window manager. Ah, the good old days ...