Since working through my issues scattered here in this forum over the last month or two, I can report
1. I NEVER use USB Sync, just the Sync button on the main page of MMA
2. Working from tips passed on by Rusty, I avoid establishing any Sync relationships from the Windows end (thus dodging bugs and confusion that have not yet been ironed out)
As you can see from the sig, I am not using beta, just the latest released versions of the paid software.

Since doing these two things all has worked as it should and without any speed issues of note. OK, I'd expect a long first sync in your case, but as a once only - perhaps overnight.

I should add that I did start 'all over again' in the hope of clearing any residual errors from earlier attempts (because I managed to fall into all sorts of traps at first). There are important steps to doing this.
and
I only use MMW AutoPlaylists to drive the syncing. IE, setup the AutoPlaylist in MMW and then, from MMA, reach back and tick them from the MMA 'Playlist' tickbox, not the obvious button on the main page but entering from the Android 'Options' button. Presumably any other way of syning should follow these steps too - for the time being.

Would it help if I started a new post to describe this in detail so that other punters might see it (rather than it being associated with artwork)??
NB: I have successfully added artwork (at MMW) to existing synced files . . . it ends up on the phone in no time at the next sync.

MMA doesn't need to be running during a USB sync, it will need to be running during a wifi sync as it's syncing files.

I am not sure what you mean by "Am I supposed to have MMA open at the time of doing the sync perhaps?" but ... yes MMA needs to be running to sync.

I only have a 64gig card, though fully loaded.
I pressed sync (wireless) on my phone at 07:44
it did the meta then analyzed then sync'd.
It updated 49 tracks and ended at 07:45

So .. the process seems to be pretty quick. Though as I said it is just 64g

Even though the sync added 1 new track. Am I supposed to have MMA open at the time of doing the sync perhaps? I do have 100GB of music on an SD card within the phone. I was just hoping MMA would retain the existing album/artwork links and only have to go through the process of attaching artwork for the new tracks added.???

It's likely that someone else can offer a much more learned answer . . . but, most of my artwork problems vanished when I followed the suggestion of another member to reduce the size of the artwork.

Since then I have ensured that just one piece of art is inserted to the track files and that is first shrunk (I use IrfanView) to less than 500 x 500 pixels, then saved as a jpg with 80% compression. Such files are well below 100KB and only takes a few moments to do. Until I checked I was adding files of several MB!

In my case I have always used the same art file name, and left it in the album folder, so a simple search and sort showed me the biggest ones, and I just fixed the worst of them, plus any that did not show up in MMA.

Hope this helps,

I suggest you send a debug log from MMA (from About screen) when this happens (add logID here for reference).

MMW is 4.1.6.1736 MMA is 1.1.1.0412

I open MMW then connect Sony Xperia Z3. I do this without having MMA open on the phone. Then sync. Disconnect and then open MMA on the phone. This is when it takes a very long time to attach artwork to each album. Even though the sync added 1 new track. Am I supposed to have MMA open at the time of doing the sync perhaps? I do have 100GB of music on an SD card within the phone. I was just hoping MMA would retain the existing album/artwork links and only have to go through the process of attaching artwork for the new tracks added.

Which version of MMA (and MMW if used) and how were files synced to your device?

After Synchronising with PC it takes over an hour to reload all artwork for all albums when MMA is opened. This is even the case where the sync only adds 1 song to the android phone. Why wouldn't existing artwork remain unchanged and MMA only process artwork for the one added track?