I would reboot your powerline adapters and if that does not improve the speed then I would recommend trying different ethernet cables.
Additionally on the sources change it does not look like it was done optimally. The sources.xml is only used for scraping. Any items already in your library will not automatically update to use the system mount just by changing the source path, only newly scraped items would pick up the new path. With the way you did this didn’t you get everything in your library in duplicate? The proper way would have been to either leave the source as it was and use path substitution, or in Kodi go to the source, context menu, change content, set to none, OK, say yes to removing all items from that source from the library, then change source path (you can do this in Kodi), then set content and scrape fresh. The latter method will lose watched status.