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Specific impulse is thrust per unit propellant flow, expressed in seconds by dividing by standard gravity. 845 kN at 287 kg/s gives an exhaust velocity of 2944 m/s and an Isp of 300 s - typical of a kerolox first stage. Hydrogen-oxygen reaches 450 s and ion thrusters exceed 3000 s, which is why they dominate deep-space propulsion.
Effective exhaust velocity
ve = thrust / mass flow rate
Specific impulse
Isp = ve / 9.80665
Because dividing exhaust velocity by g0 removes the unit system, so the figure is the same in metric and imperial - a historical convenience.
No, often the opposite. Ion engines have superb Isp but milli-newton thrust, so they take months to do what a chemical stage does in minutes.