When using a mosaic, GranuleDescriptor creates a low level reader and sets its input, causing it to perform some inizialization.
In NetCDF it is expensive.
The stand-alone NetCDF reader does not suffer from this problem because it keeps a single reader initialized only one time.
Make the initialization faster
See if we can avoid the axis loading when passing "initMetadata" set to false
Finally, can we cache the readers somehow?