Hi, this notification appears on the Samsung devices equipped with the ““Samsung Smart Manager””. Let me explain how it works:

The Avast Mobile Security app is designed to always run in the background (if it does not run, then no shields are be active and the viruses can freely pass through). And, if something “kills” the app, it immediately restarts itself. The “Samsung Smart Manager” is able to recognize this and thus may display that the app “slows the device down” while it “persistently runs”.