You can adjust warningTime in the config to avoid seeing those messages. The default is 10 and you could change that to 15 or more (100). Sounds like the server may be struggling a bit. SO MANY plugins use the same hook for managing damage including decay. I don't have access to BetterNpc or the other one to investigate further.
However, you could try this edit to the plugin. Find the lines that read as follows:
private object OnEntityTakeDamage(BaseCombatEntity entity, HitInfo hitInfo)
{
// Puts(entity.name);
if (!enabled) return null;
if (entity == null || hitInfo == null) return null;
if (!hitInfo.damageTypes.Has(Rust.DamageType.Decay)) return null;
Comment out that last line and add this new line:
//if (!hitInfo.damageTypes.Has(Rust.DamageType.Decay)) return null;
if (hitInfo.damageTypes.GetMajorityDamageType() != Rust.DamageType.Decay) return null;
See if that helps performance any.