With a folder of this size, I think you may be experiencing the same performance issues as reported and discussed in https://forum.syncthing.net/t/is-syncthing-v2-with-very-large-folders-possible/26082. This is because even though you’re ignoring most of it, the ignored files are still recorded and present in the database, meaning that Syncthing may end up spending most of the time on working on it instead of the actual synchronisation.
If you intend to share only a small part of the very large folder, I’d suggest either splitting it into multiple smaller folders, or simply adding the subfolder to Syncthing separately and sharing it instead.