4.3 lag

I have the same issue with latest roms , idont know why its happening?

first get this app: https://play.google.com/store/apps/details?id=org.vndnguyen.phoneinfo&hl=en
then see what BL (bootloader) you have. You need to have MJ5 bootloader or newer for new roms.
Also, what specific rom you trying to flash... and did you follow all the procedures they required?
It's hard to say what your problems are without more info.

Bump!

Yes... you will need to install mj5 bootloader. It will not have knox. You can get it from many locations but off the top of my head, you can get it from here:
http://xdaforums.com/showpost.php?p=46130236&postcount=3 along with odin (scroll down to the bottom for the latest).

Extract the MJ5 bootloader zip to a location on your PC
Boot into download mode on Note2 and on PC open Odin as Administrator (plug your phone to usb cable).
- Click the Bootloader button, browse to MJ5 bootloader and select it.
- Click the "Start" button and wait for the phone to reboot.
After that.. Reboot into download mode again and you should see something like "KNOX WARRANTY VOID" - you are on the new bootloader.
I would then Reboot into custom recovery and re-install your current 4.3 rom (unless you want to try another one, then follow their instructions). This should stop you from having FC issues.

*Hope it works out for you. Good luck.