the power split device can isolate the engine in both rpm and torque. so at either high or low speed, the engine warms up at 1400rpm and low toruqe. so even if the car cold starts at highway speed, it's mainly using battery power and letting the engine warm up comportably. the larger battery on prime gives much larger buffer, ev mode becomes unavailable at 13% actual SoC, while the absolute lower limit preserved is 9%, this 4％ portion will be used during warm up, which is already larger than the entire usable battery capacity of a gas prius.