Timing analysis in EDI

Hi ,

      I am facing some problems in Timing analyis of the encounter digital implementation.

      I have imported my design. Configured the MMMC for typical librarries.And floorplanning, placement and CTS and Routing has been          done as per the flow.

     And I am chacking the timing report at each step.

     When I put the clock frequency as 50 MHz in then the WNS (Reg to Reg) after optimization is coming -3.5ns.

     Then I reduced the clock frequency to 40 Mhz and 20 Mhz , then also the WNS (Reg to Reg) is coming around -3.5 ns only.

    In both the cases the MMMC is same.

   How come the WNS is same for all the frequencies.

    Can some one tell me where can be the mistake and suggest me how to improve the WNS.

Regards,

Hari Prasad