Clock is ticking past 30mins now on this 200Mbps line!! I think the Internet Pixies are currently hand crafting the pixels.
I would say it is worth noting the removal of old artwork on the ticket. Can be surprising the things that can lead to an issue occurring. Though I know I have uploaded artwork and deleted older lower res versions before myself. So it may not be that simple.
A test without the artwork option would be interesting. (So would maybe a “small” artwork test)
There have often been threads pop-up about Picard getting caught up on long downloads… but they always seem to peter out without any real solutions being found. Usually they are based around someone tagging 1000 albums. And normally that slowdown is at the writing the tags stage. Here the issue occurs just on the download of the info from the database.
And I am glad to help and be a sanity check here. Very used to testing and seeing oddities.