I’ve been running tests on iOS 15 voice-to-text dictation. The below video demonstrates an iOS 15 vs. an iOS 14 comparison on two iPhones with the same processor. iOS 15 is slightly faster and more accurate. Because it is being done on-device, you are also freed from the whims of your Internet connection.
Where iOS 15 really pulls ahead is the removal of the blasted timer. No longer do you have to randomly restart dictation as you dictate. This will be a game changer when it releases. The below video shows demonstrates this difference.