[Samsung Galaxy Nexus] ph4zrd:
The app isn't made for fixing this problem. It is made for overwriting free space on the sdcard.

So of course this doesn't fix it. The mount option does and is absolutely essential.

By simply using Forever Gone you only tried how bad this can get ;)

This might temporarily help (a wipe of the system, cache and userdata partitions does), at least a little bit.

But it doesn't fix the problem. The phone will slow down again.

Step-by-step guide

1.

Make sure you have a eMMC chip of type V3U00M using this or this app as I mentioned above.

Otherwise it might be possible that the following commands HARD BRICK your phone.

You can also use the following command: cat /sys/class/block/mmcblk0/device/name

2.

Make sure your phone is rooted.

3.

Install and run this app.

This temporarily solves the problem, the phone will eventually begin to slow down again.

Type the following commands in a Terminal or preferably an adb shell:

[strike]su
mount -o remount,discard /data
dd if=/dev/zero of=/data/tmp.bin
rm /data/tmp.bin[/strike]

It WILL take a long time (up to 3 hours maybe) and your phone might soft-reboot like mine did. Therefore using adb would be better... Some users also reported a speedup when this is done in recovery mode (CWM/TWRP) via adb.

The dd command seems to fail, issuing a message similar to "no space left on device". This is normal as we don't give the command a particular file size so it fills the partition until no space is left on it. (That's what we want to do)

This temporarily solves the problem. After you reboot the remount option is lost and the phone will begin to slow down again.

Btw, you can also use the Forever Gone app instead of the dd/rm commands. So if you don't want to use the command line, apply Step 4 first (using Script Manager), reboot and then run Forever Gone.

4.

To prevent this, add the following code to a script and put it in the init.d folder (if your rom supports it) or use an app like Script Manager to run it on every boot. (Make sure it is started with root privileges!)

#!/system/bin/sh
mount -o remount,discard /data
mount -o remount,discard /cache

5.

Profit.

The only phones affected seem to be Galaxy Nexus GSM (i9250) models with the the V3U00M chip for which it is apparently essential to get TRIM/erase/discard commands or whatever for every single block deleted.

The only reported production dates of bad chips are 08/2012, 09/2012 and 10/2012 until now. Maybe chips produced earlier and/or later are also affected.

You can get the production date of you eMMC chip using the following command: cat /sys/class/block/mmcblk0/device/date

Additional information

Google Code Bugreport (already posted above)

Galaxy Nexus Storage Performance Over Time & TRIM

[Q] Why is my Nexus so sluggish?

My Galaxy Nexus is being very laggy...

[Q] Very slow and laggy Nexus

Other guide, some FAQ + patched kernel (for a specific ROM)

LagFix Tool

some eMMC chip types used at the beginning of 2012 (not affected, type VYL00M for GSM Nexus)

Benchmark of this chip on low-mem conditions

Maybe someone can make a flashable zip or something like that for automating this.

If enough people report this problem I might also do it.

I am not responsible for killing little kittens or your girlfriend getting pregnant by that.

Cheers!