Root was removed with the OTA 4.2.2 update but the boot loader remained unlocked.
Anyway lately I had been noticing that the phone was becoming very sluggish with the camera taking 3-4s to come up, bringing up the task manager taking 4-5s and pressing Home showing just the wallpaper for a few seconds before the icons would appear.
After researching XDA and some other places it seemed like the GN and some other Samsung phones do not implement the flash memory Trim command and flash memory can slow down dramatically over time.
The sledgehammer approach is to factory reset the phone and try to keep free memory around 4GB (I had about 2GB free) but that seemed a bit over the top for me.
I then discovered that the memory could be Trimmed by an app in the Playtore - lagfix.
It does require root but since my phone already had a unblocked bootloader and custom recovery, it was easy enough to copy the latest SuperSU to the phone, boot to Bootloader Mode and flash the file.
After that running lagfix has made a major different. The phone feels like it's new again and I have dampened my hankering for a handset upgrade.
As usual YMMV but it's worth considering if you a Samsung Android phone and are experiencing lagginess. My son rooted his Nexus S and ran lagfix and that brought his phone back to life.