On X10 II (or anything with < 8+ GB RAM), main problem is running into swap and out of memory Low memory, apps crashing - change zram settings or add swapfile? [4.x]
Basically it’s impossible to have Browser (by far the heaviest app) open with some core Android work apps in parallel, everything grinds to a halt.
When I compare UX on stock Android on Xperia X (using it for Fitbit etc BT stuff) vs SFOS Android on X10 II, ancient Xperia X wins in responsiveness by a long shot.
Let’s look to keep performance issues in their own topics, though.