You can try this
1. Check for old (huge) clock buffer delay in the clock path and fix it by reworking on the Clock Tree.
2. Apply a tighter (network) latency constraint.
Can u please give me a clear and detailed idea about what u really meant by cutting long nets and add buffers?
Actually i want to know is there any relationship between cutting long nets and adding buffer and how will we get advantage bu cutting long nets??