In fact the Jet Engine thrust is a little bit less at high speed, so the reverse thrust as well.
We can assume the produced reverse thrust is more or less same througout the landing roll. However at high speed due to lift, the efficiency of brakes are less. For the desired fixed deceleration rate at landing roll (e.g MED Autobrake Selection on an Airbus FBW), most of the deceleretion job is done by T/R at high speed moments.
The following figure is not based on speed but based on stopping distance, it may give a good idea the role of deceleration methods.
Edit reason: editorial