Are there any errors in the log? There's no such limit as far as I'm aware, I have pasted much more that that (it got terribly slow though but was not truncated).
Android 6 suggests that your device is fairly old? Maybe it runs out of memory or something like this.
Can you try the build from this comment: Editing a long note is slow on Android - #48 by roman_r_m and see if there's any difference?