My experience is that SFOS has become slow, and sometimes non responsive or at least very sluggish after upgrading from v 4 to v. 5.1. It is like a bad version of iOS 26. Has there been any major changes to how the touch screen is working, or other parts of the system, that can be the cause of this behaviour?
With v. 4 I found SFOS a joy to use. So very smooth and relaxing.
For me, 10 II, every SFOS update seems to keep my system as smooth if not smoother.
I guess each person’s hardware is a thing that have memory, history… Phones get older differently. Did your storage got regularly full, did you phone overheated a lot, did it fall a lot, and so on and so on…
I don’t know my phones history. It could well have been damaged in it’s previous life. I have basically just started using mine on a daily basis two days ago, and I got it in December last year.
I think the main issue is the Android App Support;
the native apps are snappy, even on my old xperia 10 ii, but not the Android ones.
On XPerial 10 V, on the other hand, all of them were snappy. Oh well, lets see for the J2 
Yes, you are right. It’s only the Android apps. Well, let’s just wait and see with the next update.
What does it say? Android apps have more hardware payload than native apps. Look at how much install memory an Android app consumes and how large the “app cache” is growing over time. What’s the conclusion here? Try to get rid of as many Android apps as possible—that should be the way. You didn’t come to Sailfish OS to reinforce your old bad habits.
Between 4 and 5.1 probably there was the AAS update from 11 to 13 included. For this step I also observed decreased performance for Android apps
I can provide exactly the opposite experience. Since update to A13, the AAS works snappier on my x10 III.
Still I do have only 11 android apps installed and only 1 of 'em all running in background.