Today I got an iPhone 14 Pro Max and I am using my old iPhones charger (XS) to charge it. The battery is holding really well but it’s taking a long time to charge. It took about 15 minutes to charge from 87% to 88% (Using while charging). I turned off optimized charging but it didn’t help.
If the XS charger is one of the old 5W ones that used to come with iPhones, that would explain it (sorry, I’ve forgotten exactly when iPhones stopped coming with 5W chargers). You have to grab a magnifying glass and read the fine print on the charger to see the specs.