The original iPhone is the perfect size for my hands and thumb typing.
I was quite fast in landscape due to the placement of everything and my ability to also rock the phone while I typed. This increased my speed dramatically.
Sadly, that is not really an option anymore as I gladly give it up for the larger real estate of the 6" models.
If I have something long to say and I'm not too concerned about accuracy, I just use text to speech.
I don't know if it's cultural, but as someone of the 'current generation' (Gen Z), I feel that the opposite is true and typing speeds increase as you get younger (to some limit).
I've tried to back this up and only found a single, questionable, data source[0] on the topic but I do attend several typing competitions and some of the young ages are phenomenal. I was beaten a few months ago by a 15 year old who was typing >220wpm+ for 5 minutes.
I'll second the other Gen z person in the replies and say that lots of us hate thumb typing and if we're going to generalize across every single living person by one factor, my experience is that typing speed goes down with age.