Note: As a rule, use direct rendering unless you're using pixmaps. If you ask for direct and your DISPLAY is remote, the library automatically switches to indirect rendering. Here's some more information about indirect rendering:
In indirect rendering, OpenGL rendering commands are added to the GLX protocol stream, which in turn is part of the X protocol stream. Commands are encoded and sent to the X server. Upon receiving the commands, the X server decodes them and dispatches them to the GLX extension. Control is then given to the GLX process (via a context switch) so the rendering commands can be processed. The faster the graphics hardware, the higher the overhead from indirect rendering.
You can obtain maximum indirect-rendering speed by using display lists; they require a minimum of interaction with the X server. Unfortunately, not all applications can take full advantage of display lists; this is particularly a problem in applications using rapidly-changing scene structures. Display lists are efficient because they reside in the X server.
You may see multiple XSGI processes on your workstation when you are running indirect rendering OpenGL programs.