It's going to be slow for things like searching and the web interface, but the software itself is more to blame than the NAS. It's already crazy slow at times doing that on fast hardware as you've noticed (I have about 40k tracks).

Using a software-based controller with its own cached copy of (some) metadata and artwork is going to dramatically improve responsiveness for general browsing/control if not for searching. At the end of the day one will spend more time listening and that won't prove to be an issue at all.