MagSafe is supported by all iPhone 12 smartphones, but it is now known that the new charging options will be limited when working with the iPhone 12 mini. The fact is that all other devices in the family will support charging with a maximum power of 15 W, while the compact flagship will be able to receive no more than 12 W of energy. Given the fact that the compact iPhone 12 mini will be equipped with a less capacious battery than its counterparts, it is unlikely that it will take longer to charge them even at lower power. 

It should be added here that the power of the supplied current through the MagSafe charging will depend on a lot of factors. For example, when connecting accessories to the Lightning port, such as the EarPods, the charging current will be limited to 7.5W. 

Apple says the MagSafe charger must be connected to a power source before placing an iPhone on it. This procedure allows the charger to verify that it is safe to supply maximum power under the given conditions.