Filesystem notifications would be great, but they don’t work across NFS. This leaves you with full, periodic scans. For a large tree that’s a lot of metadata to fetch, and over NFS this is quite slow. The NFS client has metadata caching that you may be able to tweak (enable or prolong the cache time). You might even get better performance by doing more frequent scans, if that means the metadata is kept in cache as opposed to timing out between every scan.
Better yet, if you can, is to run Syncthing on the box that actually has the files, instead of over NFS.
Ignoring files won’t help. If you can ignore large parts of the tree, like directories at the top level, that would help as there would be less to scan. Don’t use any negative patterns (beginning with !) as Syncthing will then need to do a full scan regardless of the other ignore patterns.