A system for generating an output radiation treatment plan for a treatment volume associated with at least one target, each of the at least one target being associated with a respective prescribed radiation dose, the system comprising: a data processor configured to: calculate an updated treatment plan which when implemented by a radiation therapy machine is configured to cause the respective prescribed radiation dose to be delivered to the at least one target, the updated treatment plan requiring an updated delivery time (T UTP) to complete, characterized in that: the data processor is configured to: [i] receive a numerical value designating an upper bound on modulation efficiency of the updated treatment plan, [ii] receive a current treatment plan, [iii] calculate a current delivery time (T CTP) for the current treatment plan, [iv] calculate an un-modulated delivery time (T UM) for the current treatment plan, [v] calculate the updated treatment plan by executing an optimization process while satisfying the upper bound on the modulation efficiency, and generate the output radiation treatment plan based on the updated treatment plan, wherein the un-modulated delivery time (T UM) is calculated based on an un-modulated delivery time per field of the current treatment plan, each of which is a time required to deliver an un-modulated field (F UM) with an exposed fluence region equal to an exposed fluence region for the field of the current treatment plan, a fluence inside the exposed fluence region of the un-modulated field (F UM) having a constant value being scaled such that an average fluence is equal to an average fluence inside the exposed fluence region for the field of the current treatment plan, the exposed fluence region being a sub-region of a beam plane to be exposed by an opening between a set of collimator leaves (400-1, 400-2,..., 400-(n-1), 400-n) in a multi-leaf collimator of the radiation therapy machine during delivery of the field, and wherein the upper bound on the modulation efficiency represents a quotient between the updated delivery time (T UTP) and the un-modulated delivery time (T UM), and the upper bound on the modulation efficiency is satisfied if the modulation efficiency is less than or equal to the numerical value. calculate an updated treatment plan which when implemented by a radiation therapy machine is configured to cause the respective prescribed radiation dose to be delivered to the at least one target, the updated treatment plan requiring an updated delivery time (T UTP) to complete, characterized in that: the data processor is configured to: [i] receive a numerical value designating an upper bound on modulation efficiency of the updated treatment plan, [ii] receive a current treatment plan, [iii] calculate a current delivery time (T CTP) for the current treatment plan, [iv] calculate an un-modulated delivery time (T UM) for the current treatment plan, [v] calculate the updated treatment plan by executing an optimization process while satisfying the upper bound on the modulation efficiency, and [i] receive a numerical value designating an upper bound on modulation efficiency of the updated treatment plan, [ii] receive a current treatment plan, [iii] calculate a current delivery time (T CTP) for the current treatment plan, [iv] calculate an un-modulated delivery time (T UM) for the current treatment plan, [v] calculate the updated treatment plan by executing an optimization process while satisfying the upper bound on the modulation efficiency, and generate the output radiation treatment plan based on the updated treatment plan, wherein the un-modulated delivery time (T UM) is calculated based on an un-modulated delivery time per field of the current treatment plan, each of which is a time required to deliver an un-modulated field (F UM) with an exposed fluence region equal to an exposed fluence region for the field of the current treatment plan, a fluence inside the exposed fluence region of the un-modulated field (F UM) having a constant value being scaled such that an average fluence is equal to an average fluence inside the exposed fluence region for the field of the current treatment plan, the exposed fluence region being a sub-region of a beam plane to be exposed by an opening between a set of collimator leaves (400-1, 400-2,..., 400-(n-1), 400-n) in a multi-leaf collimator of the radiation therapy machine during delivery of the field, and wherein the upper bound on the modulation efficiency represents a quotient between the updated delivery time (T UTP) and the un-modulated delivery time (T UM), and the upper bound on the modulation efficiency is satisfied if the modulation efficiency is less than or equal to the numerical value.