I would like to know if MeteorJS's mini mongo (just to be clear, on the client) retrieves its data from memory (RAM) of from disk (maybe local storage). Also if it write to disk the objects received. I'm concerned about performance (and app responsiveness).
It is all in-memory. Thus very fast and recreated on every complete app reload.
Sometimes you want more persistent behavior and this is where packages come into place. An example of what you can do can be demonstrated in ground:db package. This package lacks good conflict resolution but one can imagine building an advanced version of it.