Part of the problem with this issue is that the symptoms are very similar to other, older problems that have been cured by things like deleting the iPod Photo Cache or reinstalling iTunes drivers. So people see certain keywords and think, "Oh, that's the cache problem" and suggest it as a solution without confirming.
This problem is new, even though the symptoms are similar, and the old fixes simply don't work in the same way.
As I understand it, when you first sync photos to a device with iTunes it will create optimised versions of each photo for the specific device. As well as writing to the device, copies of these photos are added to the iPod Photo Cache in the root photo folder in the form of Txxxx.ithmb files. Some file viewers will actually display these from within Windows Explorer or, I'm guessing, Mac OS Finder.
For reasons known only to Apple, these files are not written sequentially to a single directory but are interleaved within a series of subfolders called F01 through F50 within the iPod Photo Cache folder. There doesn't seem to be a pattern to where these files are written. An experiment I performed with an erased cache folder and the sync of a single photo showed the associated .ithmb file was created in the F04 folder. This makes it very difficult to track exactly what iTunes is doing when it's "Reading photos from...", even with a file system monitor program.
This process is very slow even on my 4.3GHz i7 PC, with an average of 2 or 3 photos processed per second. With a library of 6000+ photos, it takes a long time to sync all of them. (iTunes also occasionally stops half way through after reporting "Cancelling sync", for unknown reasons. But I suspect that's a separate issue).
Once the device is synced, subsequent sync operations compare the contents of the iPod Photo Cache with the files on the device so that only new or changed photos need to be synced. At least that's the theory, and is how it has appeared to work in the past.
With the current version of iTunes, what appears to be happening (although it's a best guess) is that the comparison phase is taking as long, if not longer, than the initial cache creation in the first place. I have left my system "stuck" on "Reading photos from..." overnight and can confirm that, despite iTunes appearing to have crashed, it will eventually get to the stage where it copies new photos over. But it gives no indication in the meantime that it's doing anything. And it takes forever.
This is why the old trick of deleting the iPod Photo Cache appears to "cure" the problem; with no cache folder for comparison, iTunes gets straight down to the task of recreating it while copying all of the photos to the device. But the end result is no faster. All that is happening is that iTunes is recreating the cache (with visible feedback, because it's also copying files to the device) rather than comparing the existing one in silence with no feedback.
Whatever the underlying mechanics, something has definitely broken with one of the recent updates either to iTunes or to iOS 11 or possibly a combination of the two. Copying 6000+ photos to my devices was always a slow process, but adding a handful later was never this slow. Now it seems I have to choice but to wait literally hours for the comparison or recreation of the iPod Photo Cache to complete for all 6000+ photos just to add a couple more.
FWIW I first saw this problem on an older iPhone 6, and I had initially put it down to the age of the device and the possibility of file corruption from the numerous failed syncs and/or reboots it's had over the years. But I'm seeing the exact same thing on two brand new 6th generation iPads.
For a while I thought it might even be a Windows indexing thing, so I turned off indexing for the folder where the iPod Photo Cache is created. This may have helped a tiny bit when it comes to copying speed, or it could just be wishful thinking on my part. Either way I've left it off just in case, but it's far from being an answer.
It's so frustrating, but for now the only workarounds seem to be either deleting the iPod Photo Cache each time and accepting that you're going to have to watch all photos both old and new get copied slowly to the device, or simply walking away after starting the process, and accepting that iTunes will eventually get around to copying the new stuff.
As an aside: sometimes, in my more paranoid phases, I wonder whether Apple isn't deliberately breaking legacy features in an attempt to wean more people onto the iCloud Photo Library. Their long-term goal seems to be removing the need for a PC- or Mac-based host for phones and tablets, doing everything through iCloud instead. I can't speak for everyone, but that's definitely not going to work for me. I take most of my photos on cameras other than Apple's, and I need the ability to synchronise folders full of photos from various sources. Until iCloud Photo Library can accommodate those needs, it'll be iTunes sync all the way for me. Unless Apple breaks it completely, in which case I'll just look for a third-party solution. For now, iCloud isn't it.
