the c to lightning cable support fast charge, you can use back your old iPhone 6 cable... if your charger adapter can output 5V 2-3A, by using back iPhone 6 cable able to draw 10-12W as well... not really fast, but is faster than 5V1A 5W
the cable usb c to lightning support Power Delivery (PD), by spec it suppose can let newer generation iPhone draw ~18W, thus, Apple call it fast charge

you might notice a lot other Android maker mentioning charger they will stated Watt, e.g. 30W, 50W... fast charge
this year iPhone 12 lineup seems able to fast charge at 20W, probably 9V ~2.2A or 12V xA maybe...
Apple fast charge is miles behind many android maker which uses proprietary charging method because Apple is using USB PD... which use commonly in modern laptop
conclusion is, if you buy new iPhone 12 any model, without buying a USB C Power Delivery support charger, it wont fast charge your iPhone 12

USB PD charger nowadays reliable brand is cheap, <RM50, some can get RM25-35