When performing a major upgrade to one of a car’s components, you can choose to pay a Gold cost to skip the time duration that must elapse between starting the upgrade and the upgrade completing. The Gold cost is greater for higher rarity cars, and for higher levels of upgrade (i.e. skipping the time to upgrade a component to 3.0 costs more than to skip the time to upgrade a component to 2.0).
After 25% of the upgrade period has elapsed, the Gold cost for skipping its duration will begin to decrease. Rounding up to the nearest 10 Gold, the price is reduced by the proportion of the time that has elapsed - so for example, after 70% of the fusion period has elapsed, you can choose to pay just 30% of the initial ‘skip’ cost to skip the remaining duration. This may be useful if you find yourself in need of a particular upgraded car in a time-sensitive situation!
After 25% of the upgrade period has elapsed, the Gold cost for skipping its duration will begin to decrease. Rounding up to the nearest 10 Gold, the price is reduced by the proportion of the time that has elapsed - so for example, after 70% of the fusion period has elapsed, you can choose to pay just 30% of the initial ‘skip’ cost to skip the remaining duration. This may be useful if you find yourself in need of a particular upgraded car in a time-sensitive situation!