This utility notably decreases the startup time of your X sessions, provided
that you start a number of X clients automatically during the X session startup.
Most people, for instance, start X clients like xterm, xclock, xconsole and
xosview from their .xinitrc, .openwin-init, .xtoolplaces or .xsession file.

These X clients are started simultaneously (in the background) which puts a
high load on the X server and the OS:

* The X server is not multi-threaded, so all X clients are competing to get
  access to the X server and to use its resources, which causes a lot of
  overhead (= delay).

* The performance of other (non X related) tasks served by the system degrades
  badly due to the high load.
  If the system has not enough RAM to hold all the X clients, it is swapping
  heavily, resulting again in a lot of delay.

On the Sun platform there is a utility called 'toolwait' which solves these
problems: it starts one X client in the background, waits until it has mapped
a window and then exits.

Xtoolwait is a free implementation of exactly the same idea.