iPhone 15 Pro Max to iTunes Backup over USB-C taking 24+ hrs

As other apple users have noted, iPhone backups to iTunes started to take a very long time a couple of updates ago. All of the threads I could find end with someone listing some basic checks and closing the thread, which isn't helpful in my situation. I've backed up all my iPhones to iTunes since my iPhone 2 so I'm very familiar with the process.

My set up is as follows :

• iPhone 15 Pro Max 1TB with 478GB free storage. iOS 26
• iMac 27" on Mojave with 72GB Ram
• 4TB Samsung T9 Portable SSD with USB-C
• Original 60w woven 1m USB-C cable for the iPhone - iMac connection

Backing up to my external device works and has worked well. It's just painfully slow with first-time backups easily running over well 20 hours. This makes no sense given the possible transfer speeds between the different devices. iTunes also decided to do a first-time backup after I renamed my device. Sigh...

iPhone content

• around 50 apps
• 40k photos (primary use of storage), 370Gb
• 3000 short videos
• 22Gb Music

Like many other changes by Apple, this is very annoying. There is no obvious reason for why it should be this slow so it feels like Apple is actively making this a worse experience to push people to iCloud subscriptions. I have zero intention of putting all my photos and data there and if this is the only way forward, I will have to stop buying iPhones. This is not a thing I take lightly after being a huge Apple fan for 20 years.

Any help is appreciated.