[black_eagle]

I tried using folder type nodes, but don't know how to restrict them by path?!

If you've added different sources to your library, you can use a folder type node to pull out individual sources but you need to know the sourceid from the database (you can get it by navigating manually to the source you want, and looking in Kodi's debug log).

For instance, I have added a source for stage musicals and all the albums for that are in a specific path.  To get that source in a folder type node, I do this.

xml:
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<node order="150" type="folder">

    <label>Stage Musicals</label>

    <icon>DefaultMusicSources.png</icon>

    <path>musicdb://sources/30/?sourceid=30</path>

</node>

That then takes me to all the artists in that source, and from there to albums.  The visibility condition that you were asking about is simply to ensure that nodes are not displayed if there is no content to fill them with.  E.g. The compilations or boxset nodes are not present if you don't have any compilations or boxsets.

You can also do it this way (which is easier and takes you to albums not artists)

xml:
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<node order="150" type="folder">

    <label>Stage Musicals</label>

    <icon>DefaultMusicSources.png</icon>

    <path>musicdb://albums/?source=StageMusicals</path>

</node>

I guess it all depends on how your stuff has been added.  If you've added a source of say, comedy and another of AudioBooks then you should be able to pull out the contents of those sources in the same way.  BTW, it's not the size of the db that's the problem with filtering, it's the way Kodi itself builds the filter(s).  I know work is ongoing though to change this to be done in the db, which should be much faster.  There is already a PR for the initial part of the changes.