This is the object that represents the association between a note and a tag - essentially it contains a note ID and a tag ID.
With nearly 150k objects you do have a very large note collection so that could be the reason why it's slow. The WebDAV format is relatively inefficient for sync because we have to download the complete object list (not their content, just their path) in order to compare and sync with your local note collection.
Downloading the list probably doesn't affect the app much but parsing this giant XML file probably does. So maybe that's the slow down you were experiencing - to make it happen less often you can increase the sync interval.