Hi @exic
The APK is here: https://github.com/roman-r-m/joplin/releases/download/input-lag-test/app-release.apk
Unfortunately, you'll have to uninstall Joplin fist before installing from the APK. This is because each package is signed by a private key; I obviously don't have Laurent's key so signed with mine. But Android won't let you install the same app signed with a different key. Hope this is not too much trouble.
If you only try on a single note (without syncing all your library) and do not see the lag, it would be great if you could also check when all your notes are synced. I want to make sure the lag isn't caused by some background processing which may depend on the total number of items in the profile.
Btw, this version is not fully functional -- attaching files/images isn't working but I believe this is not where the lag was occurring anyway.
Thanks for volunteering. And just in case - make a backup first.