While using import and export locally I realised that export is many times slower than import which made no sense to me.
I noticed two things:
1) The progress updates are not happening at 250ms intervals as we specify here, but in fact seem to happen in a tight loop (probably every row?):
2) At least locally in development the dev tools console breaks during export of large collections. You have to close it and reopen it to get it to work again. I logged every time we update the progress and only the first few logs would show up even though the progress keeps updating.