How to increase data through put of RS485 RTU?
It is generally accepted that RS-485 can be used with data rates up to 10 Mbit/s or, at lower speeds, distances up to 1,200 m (4,000 ft). As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 10 on the power of 8. Thus a 50-meter cable should not signal faster than 2 Mbit/s.

The modbus documentation from TycoElectronics.