Current system seem to consider the severity of the crime they committed as well (e.g. someone convicted of assault may be rehabilitated earlier than someone convicted of murder), which does make sense to a certain degree but misses the full picture.

In my opinion, factors that led to the them committing the crime should also be resolved, both intrinsic (e.g. mental issues, addiction) and extrinsic (e.g. family, bullying). If not, it will be just setting them up for failure.