When the incremental flow is enabled, only new incremental-friendly suggestions can be applied in the subsequent steps. This limits the amount of change that a suggestion can achieve, so large timing improvements are unlikely to be seen.
If suggestions are applied at opt_design and the incremental
flow is initiated after, larger impact suggestions can be applied. It will likely be
necessary to disable the suggestion because it might cause timing degradation.
If the automatic incremental flow decides to use the default flow because of a poor quality reference checkpoint, all suggestions can be applied.