USB-A (or what you call regular USB) only supports older versions of the USB power delivery standard, while the charging standard on a lightning port is proprietary.

So, it could be that your charger doesn't support the level of fast charge your phone is capable of: basically, your charger is telling your phone that it doesn't know how to charge your phone super fast safely over USB, so it will try to charge it slowly in the way it knows; it knows how to charge a phone over Lightning slightly faster, so that's what it does.

Try getting a GaN based charger with at least one 45w based USB-C output port. (Anker, if you want a brand recommendation) and your phone should be able to get a much better fast charge from USB-C to USB-C.