Slack is reported with actual routed net delays except for the nets that are not completely routed. Slack reflects the impact of hold fixing on setup and the impact of congestion.
No hold violation should remain after route, regardless of the worst setup slack (WNS) value. If the design fails hold, further analysis is needed. This is typically due to very high congestion, in which case the router gives up on optimizing timing. This can also occur for large hold violations (over 4 ns) which the router does not fix by default. Large hold violations are usually due to improper clock constraints, high clock skew or, improper I/O constraints which can already be identified after placement or even after synthesis.
If hold is met (WHS>0) but setup fails (WNS<0), follow the analysis steps described in Analyzing and Resolving Timing Violations.