I need to reduce the area, change regslice ?
The only way to reduce area is to reduce the complexity of your configuration. Register slices do add area, so if you are not pushed for timing you could remove any slices that are not mandatory (the configuration tools will tell you when a register slice is needed for protocol reasons).
When you describe this NIC-400 as implementing a "big bridge", do you mean a bridge as described in the NIC-400 documents as being a simple single-input single-output bridge between two clock domains, data widths or protocols, or is this something bigger like a full interconnect with lots of connecting blocks ? If it is the latter, you would have more scope to simplify things, but that would probably be better discussed directly with the Arm support team if you license NIC-400 direct from Arm.