The warning may spawn due to how the Feature Service was added as an item in ArcGIS Online:
Be aware that when adding the Individual Feature Layer, the description, tags, and thumbnail are not populated automatically.
Unless you or other users are seeing performance issues then this warning can be ignored.
This document on ArcGIS Hub: Data manager—ArcGIS Hub | ArcGIS
States that "query performance warnings appear when the service takes longer than 1500 milliseconds to respond to a simple count query."
So this could be a just a general warning.