Largely it's for the performance and control.
On the stock rom, you will always have lag. There's no avoiding it. On custom roms, you have a lot less bloat. As you're rooted as well you get more control of the rom.
For instance, I usually run V6 supercharger on my roms. This frees up ALOT of ram. I also add entropy tweaks which makes everything more responsive.
The real problem that you're running into I think is just dirtyness.
Backup whatever you want with titanium backup. If you have anything on the phone that you actually need.
Download a CM10.1 nightly.
Download the proper gapps for it
Goo.im Downloads - Downloading gapps-jb-20130301-signed.zip
Put them on your sdcard.
Reboot into recovery
Backup & Restore > backup
Wipe Cache
Factory Reset
Install the rom
Install Gapps
You should be good to go as far as force closes.
TBH, I'd avoid kangs until you get CM working correctly. Everything else is based off of it (mostly, exception being pure aosp). If you can't get it to work without force closes.... there's something wrong. Kangs and such, that could just be an issue in the kang.