Instructions:
make sure you are rooted.
WARNING: MAKE SURE YOU ARE RUNNING STOCK ANDROID VERSION 4.1.2 (THE RELEASE, NOT A LEAKED VERSION) OR YOU WILL DESTROY YOUR DEVICE DUE TO BRICKBUG!!!
READ THIS POST IN FULL. find out which bugs your eMMC has if any, and decide whether to run the risk of trimming.
WARNING: MAKE SURE YOUR EMMC IS NOT AFFECTED BY TRIM BUG OR YOU WILL DESTROY YOUR DEVICE!!! if you have trim bug, you must not trim on a stock kernel, end of story.
also, it is assumed that release (not a leak) 4.1.2 stock kernel contains this patch and thus is brickbug safe. but there might be different versions, and there is no way to be sure if the corresponding source code was patched by samsung, so...
WARNING: IF YOUR EMMC IS AFFECTED BY BRICKBUG, THE POSSIBILITY HARD BRICKING YOUR DEVICE CANNOT BE COMPLETELY RULED OUT without access to the kernel source code. proceed at your own peril, or better yet, switch to a custom rom/kernel.
install the LagFix (free) app from xda (the market version is declared to be incompatible with some 4210-based devices). go to the LagFix tab, check the 3 partitions, and tap on run. grant root access. the 3 fstrim operations should be successful ("partition was trimmed" means success). alternatively, those with busybox installed can try issuing the fstrim commands themselves. in particular, you must do this to trim /preload. you can also look for the fstrim command in the private files of LagFix.
reboot and profit.
NOTE: i assume there is little free space in /system and /preload in stock roms, so most benefits will come from trimmed free space in /data. this space will get overwritten in time so you will need to periodically trim. the premium version of LagFix includes a scheduler that takes care of this. unfortunately LagFix is declared to be incompatible with some 4210-based devices and so you will not be able to purchase and download it. FYI, here is some information about the genuine LagFix premium apk version 1.5.1 on Android Observatory and AndroTotal, including hashes of the file. (i do not know about free space in /preload. maybe there is enough to overprovision but you need to manually trim it; LagFix will not do it.)