It's about the number of components/clickable components in the screen/each note. The more you add the worst it gets. We might have added too many. :(
Discussion
I did try to comment most of the feed components on amethyst's code and leave just a simple list and it still was not fluid enough. That's why I suspect something related with how data listener/consumer model is handled.
On my S19, I have to remove the rich text of posts + relay icons + turn most of the SVG icons into bitmaps to run smoothly. In the notification, I have to remove the clickable element of each of the author pictures. :(
Of course, if we can minimize the processing of new events, that helps as well.
I am currently refactoring the base caching system and the new one should help, but it is a lot of work.