I have a Samsung Galaxy Note II running CyanogenMod 11, so it's obviously rooted. As of recently the phone seems very laggy and much slower than it used to be. So I did some research and tried
fstrim, among other things.
However, when I run
fstrim -v /data as root (I use terminal emulator),
/data: 505982976 bytes trimmed
However, when I repeat the above command, I get exactly the same output. In my eyes this indicates the trim does not work and the system remains slow. I do the same on my Linux desktop machine and subsequent
fstrim -v commands output zero bytes trims, to me this is what I expect on Android as well.