[itimpi]

I run with a similar configuration. The use of a NAS certainly slows things down, but performance is much better if you keep the metatdata.db file on a local drive.

Calibre provides a mixture of runtime parameters and use of environment variables to control placement of the ebook files, the calibre configuration and the metadata.db file. I have attached a copy. It is reasonably commented so you should find it wasy to edit it to match your particular setup