This answer record provides a patch for the GTX/GTH models in the 14.4 ISE environment to allow for faster simulation times.
This patch applies only to the ISE environment.
This patch is necessary in ISE Design Suite if you wish to use the fast models for simulation with one of the supported simulators (Aldec, Modelsim, NCSIM, or VCS).
Unlike Vivado Design Suite, ISE Design Suite does not have the Unifast libraries already integrated in the install.
Please note that the .zip files have been broken up by simulator.
It is not necessary to install all of the individual patches, but for a complete installation it is recommended that you install all of them.