So by that I mean that I have seen from the keyboard size (which is slightly larger in the apps than for example in the Apple internal apps and many other apps from the app store) that they use some module, code or similar, like WhatsApp, which is not optimised for the iPhone 12 Pro. The assumption now is that these apps, when running in the foreground or background, can be responsible for the stuttering. The feeling after a few hours is definitely that since I make sure to close these apps immediately after use and I have deactivated the background update for these apps, the iPhone runs much smoother and feels like it has much less stuttering. I'll keep trying this during the week, but I don't want to say that this is the solution yet. I'd rather verify that for a while longer.