To properly reflect how format specific metadata (such as ID3 v2.3) will be saved the metadata view currently makes use of File.new_metadata. That rewrites the metadata so it's presentation fits how it will finally be saved (depending on file format) independent of internal representation.
The way this works currently slows down the UI, especially the metadata view. That is because new_metadata is accessed often and always creates a copy of the metadata.
In PICARD-905 Sophist has proposed an alternative implementation.