When you have a large number of albums/files loaded, the UI becomes sluggish if you select a large number of these to e.g. save.
The reason for this is the metadata analysis that is done for common tags when you select multiple items which has to loop through every metadata item (both original and new) for every track selected.
Performance could be improved by an order of magnitude if, under the covers, picard maintained such a summary at an album level so that it was looping only over albums (except where the album is expanded in the UI and not all of the tracks have been selected). Assuming 20 tracks per album, we should expect this to be 15-20 times faster than at present.
Of course, it does require maintaining the summary by album whenever the metadata for any track is changed, and you wouldn't want to do this for every individual change of metadata e.g. when the webservice call completes, so it probably needs a flag to say that the album summary needs updating and a timer triggered say (100ms) after the first metadata change - so the technicalities are non-trivial.
However, this would be a useful improvement to include in Picard 1.4.