A big part of the N9/50 feeling slower after some time is the flash memory getting dirty.

Flashing solves this, but that's quite the dirty hack.

IIRC, while working on some block stuff for kernel-plus, I saw some stuff patched in for preliminary discard (TRIM) support - was never finished by Nokians.

That's pretty much the main cause of percieved slowdowns - pretty much no packages even bother to jump the hoops to autostart, that's a variable gone.