One of the hardest problems for the Interstellar Club to solve was not propulsion or navigation, but rather time. Even at a large fraction of the speed of light interstellar travel is measured in decades. An ego sleeved into a Neumann morph must be expected to spend most if not all of its life in isolation, with lags of years on light-speed communication, in the sensory deprivation chamber of interstellar space.
The subjective run time cyberbrain modification ties the clock speed of the cyberbrain to the amount of stimuli received by the senses. An expert program continuously evaluates received information, increasing or decreasing the speed of thought to match. To an interstellar traveler, the long spans of interstellar travel would pass relatively quickly, with the bulk of its mental life occurring while inside solar systems, making observations, or while receiving communications.
Some couriers and ring-fliers have been experimenting with using the mod; there are few other potential in-system uses. The program is open-sourced through the Argonauts.
Subjective run time is cyberware [Moderate]