If you enable software monitoring then your audio will be "off" with the amount of latency that you hear. Meaning if you play so that what you hear through the software monitoring is correct then it will be correct later too. If you disable software monitoring it should record the audio at the moment you play it (relative to what reason plays at that moment). So it should be correct if you have direct monitoring though your interface.
Reason can only really control its own processing buffer, which should be small enough at 128samples. What the audio interfaces driver etc. add is outside the control of Reason. With software monitoring no audio interface really gets below 8ms real latency.
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Professional Audio Engineer at Theater Bremen, Germany • Part Time Programmer