1. A method of performing physical synthesis using persistence-driven optimization, the method comprising: using a processor, ranking nets in a design based on unpredictability and expected quality-of-result impact; selecting a first predetermined top percentage of the ranked nets as first persistent nets; performing timing-driven global routing on the first persistent nets; back-annotating a timing graph with actual delays and parasitics determined by performing the timing-driven global routing on the first persistent nets; running synthesis for the nets in the design using the actual delays and the parasitics for the first persistent nets, wherein the synthesis maintains and updates routing for the first persistent nets; re-ranking the nets in the design after synthesis based on unpredictability and expected quality-of-result impact; selecting a second predetermined top percentage of the re-ranked nets as second persistent nets; performing timing-driven global routing on the second persistent nets that had not been selected among the first persistent nets; performing global routing on the nets of the design while maintaining existing routes of the second persistent nets; and outputting a final layout of the design based on the global routing.