Your h265 example is due to the iPhone having a dedicated HW encoder while the iMac was rendering using the CPU. A hardware video encoder is almost always going to be faster and more power efficient than a CPU-based one by definition. However, a CPU encoder offers more flexibility and the possibility of being continually improved to offer better compression ratios.
Generally, HW encoders offer worse quality at smaller fie sizes and are used for real-time streaming, while CPU-based ones are used in offline compression in order to achieve the best possible compression ratios.
Generally, HW encoders offer worse quality at smaller fie sizes and are used for real-time streaming, while CPU-based ones are used in offline compression in order to achieve the best possible compression ratios.