How does the quick charge feature work on my Xperia?

When you use your Power Delivery (PD) charger under ideal circumstances (25 ° C environment/screen off state), you'll be able to charge your device in the fastest way possible: 50% after 30 minutes from 0% start. After reaching 50%, the charging current will be gradually reduced to protect the battery, so the charging speed will also go down a bit. You'll also experience a reduced charging rate if you use a low capacity charger (5V charger). When you use such a low capacity charger while using your phone, it may not charge, or the battery level will even go down. 

During quick charging, your phone might slow down to manage any potential heat build-up. You can boost charging speed by powering off your phone to avoid extra battery consumption. The charging rate may be slow or stop entirely to safeguard your phone in a high or low-temperature environment. Finally, if you drain the battery entirely and your phone is powered off for a prolonged period, it may take some time before it starts charging when plugging it in.