When your design has critical path nets with high fan-out, use the syn_maxfan attribute to control the fan-out of the net. Setting this attribute for a specific net results in the replication of the driver of the net to reduce overall fan‑out. The syn_maxfan attribute takes an integer value and applies it to inputs or registers. The syn_maxfan attribute cannot be used to duplicate control signals. The minimum allowed value of the attribute is 4. Using this attribute might result in increased logic resource utilization, thus straining routing resources, which can lead to long compilation times and difficult fitting.
If you must duplicate an output register or an output enable register, you can create a register for each output pin by using the syn_useioff attribute.