General Description: There are several pin delays, net delays, and routing delays that have been updated in 1.5i Service Pack 2, which now available on the web.
First, please note that this problem concerns a change in the speed files - not a change in the speed grade.
The early XV speed files had delays that were both slower and faster than they should have been. The Service Pack 2 versions of these files have corrected both types of errors.
When PAR runs in timing-driven mode, it seeks out the fastest way to complete each net; if a speed file has an excessively fast delay for a particular feature, then PAR will prefer that feature over others which might actually be faster.
Now, when trace is rerun with a new, corrected speed file, those routes with a large number of the "prefered" features will slow down dramatically. When you get a new speed file, you must reroute and replace the design, then compare the timing for the old and new solutions.
Simply rerunning the timing analysis with the new file will tell you the true delay for that routing solution, but it will not reflect the actual performace available for your design in using that device.