See this topic by Arthmoor on the AFKMods forum: TES5Edit - Records that merge at runtime
To quote Arthmoor:
NAM0, NAM9 subrecords: Used to determine the in-game cell dimension of a worldspace. These have been confirmed to be exempt from the Rule of One. *ANY* mod making changes to these sets of values will have those changes propagated into the game WITHOUT conflict management. Be aware of this if you are running two mods together which intentionally change these values for a worldspace. The outcome will not be predictable and you may end up with a worldspace size much larger than you're expecting. This can lead to huge amounts of stuttering in the game that have no other identifiable cause. If you have mods which are altering vanilla worldspaces, chances are these edits were NOT intentional unless the author specifically states they have done so. They can appear in a mod unintentionally and the author may not be aware it has happened.
This is important. Some mods like ELFX - Exteriors *do* change the Worldspace object bounds...doing a search on the Nexus forum topic, the author states that these edits were NOT intentional. If this is the case with just this single mod I looked at, other mods may have the same issue.
As these records do not follow the rule of one, they need to be corrected for any mods that do this (unless by chance it was an intentional edit - likely it was not). It is not sufficient to forward the correct object bounds in a patch. You must correct it so that every mod for a particular Worldspace record shows the same NAM9 and NAM0 values.