Slow with Tidal and slow with local files could be two seperate issues with similar symptoms. I think it is a good idea to focus on the local file slowness first.
When you say local, where are the files relative to the nucleus? Are they on an internal SDD/HD in the Nucleus, are they attached to the Nuclues via a USB external HD, or are they on another PC or NAS in the network?
Also, what is the endpoint that you are using when you experience this slowness? Is it wired or wireless?