Why does the Design Space Explorer report "unknown" timing results?

Due to a problem in the Quartus® II software versions 11.0 SP1 and earlier,  the Design Space Explorer (DSE) may not report timing information correctly if the TimeQuest timing analyzer is not explicitly identified as the timing analysis tool. Instead, DSE reports the timing slack for all seeds as "unknown".

To work around this problem, add the following assignment to the Quartus II settings file (.qsf)  for your project:

set_global_assignment -name USE_TIMEQUEST_TIMING_ANALYZER ON

This problem has been fixed begining with the Quartus II software version 11.1.