What is an externally referenced actor and how do I fix it?

So when I go to my message log, there are some warnings saying some of my actors are “externally referenced”.

Does this slow down my game at all, and how do I get rid of or fix it?

Hey masterburgler,

Here are a couple of post that explain what that error is and some ways to fix it.

Externally Referenced warning - what does it mean?

UE4.6.1 “Externally Referenced” on basic map

Let me know if that helps.