This topic briefly talks about it, and if I understand correctly, iCloud sync is not used when the library used is a Plex library.
In the case of the Plex library being a remote one, do you think there would be a performance gain using the iCloud sync as a first way to store data, and then updating using Plex?
Correct. iCloud Sync is a central database, similar to a Plex server.
IMHO, there is little benefit to using both, and in some cases can cause confusion. For example, if Infuse caches your library in iCloud but then you may changes on the Plex side, these changes may be delayed or not picked up at all.