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....

 

The throttle sensitivity on my 2017 Prime seems to "dull" when switching from EV to HV mode, kind of like the opposite of a "Sport" mode on other cars where the throttle takes less effort to produce the same acceleration.

This is extremely annoying when switching from HV to EV mode - it gives a sudden burst of throttle that's similar to 3rd party "trick boxes" like the Pedal Monster, Sprint Booster, etc. which is un-linear and jerky.

Why did Toyota give us a non-linear throttle map when switching from HV to EV mode? Feels like someone mapped 10% throttle to 30% throttle in an instant & it's a lurch of power. I am already in ECO mode...

FWIW, lots of people like this "twitchy throttle", go drive something like a Viper, Vette, 5.0 Stang, Challenger with a more "linear throttle" and then every 4-cylinder you drive feels like it's mosquitoes on crack. Can we just get linear throttle in every car? My HV mode is super linear, EV mode is like every other small car with an "inflated throttle"

 

NOT on a CVT transmission - every car you mentioned above has firm gear ratios and slight delays while switching. On a CVT car; the ICE ECU and transmission ECU communicate and power takes on a "rubber band" feel - NOT instantaneous, like gears.