This paper brings to the fore the key parameters set out on thermal yield and energy efficiency in microwave industrial processes, specifically, in the transient phases. It examines in detail two industrial operations: thermoset polymer curing and flow chemistry under pressure. Overall tuning systems are necessary for energy optimisation, but on the other hand, vessels, pipes, moulds and conveyor belts made of lossy materials are required for holding products or carrying materials. The impact of these inserts on the temperature profile of the heated product and on energy (the power usage and energy efficiency), were experimentally and theoretically examined. It was demonstrated that these parameters have to be considered for process design and calculations. Looking at these two examples, one can see how transient phases are crucial for the required power of the microwave generator and the process design.