Advantage to having SSD on core processor motherboard vs USB connected SSD storage?

Would there be any performance advantage to having music files stored on the same motherboard as the processor (getting one with slots for SSDs) as compared to having the storage of files on a separate device connected via USB 3? This would affect whether I get a NUC, which due to its deliberately minimal features, would not have such slots.

The kind of difference I imagine it could make would be to the speed of processing the database, and therefore how quickly searches and links respond, and the time from pressing play to hearing the sound. I also wonder about any audible SQ differences.

There is one golden rule, make sure to have a very fast disk for the program + database, and have a separate disk for music.
It is nice to have your music on a internal SSD (I do), but a decent SSD or traditional hardisk in an external housing via USB3 is perfectly fine (and maybe advisable for ease of access, backup, replacement by a bigger one, …)

Thanks Dirk. And what about the processor and RAM of the core host? I have read here that the i3 is good enough for a smallish library. I am hoping to accumulate a few TBs over time so am thinking that an i7-based NUC would be best. Since RAM is relatively inexpensive I would go for 16GB or more.

Probably slightly overkill that will never hurt in the future (except if you are very conscious about power consumption).
The size of the library is far less important, compared to eventually start applying DSP (digital signal processing).

RAM is important, my set up takes 2.5 gb of my 16 presumably loading the library into memory for quicker results, I have 150 k tracks

I use a tower PC i7, 250 ssd , 16 gb Ram , 2 x 4 Tb HDD for files , music and video

It’s my general purpose PC as well

Works fine

That’s encouraging Mike. I have now made purchases of a NUC7i7DNFE with 16GB RAM and an M.2 SSD 250GB for the core and database, and a Synology 918+ NAS with 4x4TB Ironwolf Pro HDDs. The latter will be in a cupboard under the stairs, connected by ethernet to the former, which will be next to the DAC in the audio component cabinet.