Rapid charging is quoted 20→80% because that's the fast part of the curve; beyond 80% the car deliberately slows to protect the battery.
The time is essentially the energy you're adding divided by the car's real DC power. A car that pulls a genuine 75 kW will add a 40 kWh top-up faster than one limited to 46 kW. Our journey model approximates the DC curve as a flat fraction of each car's peak rate, deliberately conservative rather than quoting the optimistic peak.
Use the journey twin on any car page to see how many charging stops a specific trip needs and roughly how long they'd take. The approximation is described on the methodology page.