Known Issue: 1.9.3
After installing the v1.9.3 patch, timing closure becomes more difficult.
This problem has been fixed in the v1.9.4 patch. See (Xilinx Answer 30124) to obtain the patch.
Even after applying the patch, timing closure might remain a challenge. In these cases, ensure the -xe c switch (continue indefinitely) is used with MAP and PAR. Also, it might be necessary to try multiple PAR seeds using MPPR or SmartExplorer to close timing. Please see the "Development System Reference Guide" in the ISE software manuals for more information on these options.
03/19/2009 - Initial Release