2018 Prius Prime - Losing Power on Freeway after EV Range is Depleted

You're asking a lot of an ICE to be able to go from stone cold, not running to 70 MPH seamlessly. Better approach is to pay attention to your EV range/SOC and select HV before running out of EV.

 

It's probably a combination of you going uphill, car in ECO mode, and demand for power at the same time. The transitions is mostly seamless, but the ECU isn't going to allow a cold engine to go from 0 to 5000 rpm in a second. You've probably also noticed the throttle being less responsive after the switchover to HV mode. It's the ECU protecting major components of your car.
It's the nature of the beast. Whenever I'm planning a trip into the Sierras, I manually switch over to HV mode in the flats to warm up the engine before the steep climbs. If the throttle response frustrates me - I kick it over to normal or performance mode. Bottom line, the engine is warmed up and ready to take on the steep grades. No lags in transitions between HV and EV modes.

You can try switching the base mode of your car to performance or normal mode to shorten the delays. You'll transition sooner but it'll cost you mpg and less EV only distances.

 

When that has happened on the highway my foot is in the same place until the engine starts. The starting takes maybe two seconds. I wouldn’t even try to put the pedal to the floor during the two seconds. My reflexes aren’t fast enough.

 

I'm the same; but some people are always pedal to the metal types....