You must evaluate the design WNS after each synthesis and implementation step. If
you are using the Tcl command line flow, you can easily incorporate
report_timing_summary after each implementation step in your
build script. If you are using the Vivado IDE, you
can use simple tcl.post scripts to run
report_timing_summary after each step. In both cases, when a
significant degradation in WNS is noted, you must analyze the checkpoint immediately
preceding that step.
In addition to evaluating the timing for the entire design after each implementation step, you can take a more targeted approach for individual paths to evaluate the impact of each step in the flow on the timing. For example, the estimated net delay for a timing path after the optimization step might differ significantly from the estimated net delay for the same path after placement. Comparing the timing of critical paths after each step is an effective method for highlighting where the timing of a critical path diverges from closure.