Yes, the “4K New” library was already fully scanned in, nothing was changed. It is just the subsequent scans that are really slow.
I did a couple of tests the other day (don’t have a log for those as I already removed the libraries), I made 2 new Anime Movie (less items to scan) libraries using the new agent.
One library had non-remux movies and the other one had remux movies, the initial and subsequent scans were fast but when I add the remux folder to the non-remux library or the non-remux folder to the remux library, the subsequent scans for those libraries will be really slow compared to the legacy one even after everything is fully scanned in.