I can report a stuttering keyboard also with a brand new iPhone 15. But I think it's a bit more complicated than that. I can also report that this is reproduceable on an iPhone 13 mini and iPhone 14 Pro.
- First, it depends on how fast you typing
- second if you use swipe-touch it's not stuttering (the reason is you are usually much slower with that)
- it seems to be a performance problem with the predictive text feature trying to keep up with a fast typer
- restarting / clear apps does not take any effect.
- It's a permanent bug
I prefer typing on single letters rather than swiping over the keyboard for generating words. If you doing this for years you are much faster than the swipe input variant (because of friction of your skin on glass). But when using swipe-input I can't recognize any lag.
So some Users won't experience this at all. Depending on speed of input and method of input-
before the predictive text feature this wasn't a problem except Apple's autocorrect is the worse I ever experienced. Now with iOS 17 it's better but it seems there is a performance problem Apple has to look into.
If you slow down in typing you will experience that the stuttering get's less. If you get up to speed again, the problem occurs. No matter which device. iPhone 15 Pro or an iPhone 15, there is actually no difference in this.
At least in my opinion this is something considered a bug. Since Apple implemented a feature that's lagging when used under speed, which is a quite common usecase for a software keyboard.