The images are on an external HDD USB3.0 (I've almost 7TB worth), the Photoshop scratch disk is an external SSD USB-C, loads of free space. Internal boot disk has 400GB free, swap is still zero.
Even so, any i/o should be 'credited' to PS not launchd, file saves certainly are. The only thing launchd should be handling is process start and termination; and
maybe it does the Rosetta translations (probably only Apple know that for certain)
I did wonder whether it was the hand-off from LR to PS so I
- Passed a .NEF from LR to PS
- Saved it as a -Edit.Tif having done the geometry in PS and renamed the background layer
- Rebooted the machine to clear any caches and 'temporary' translations
- Opened the .Tif directly in PS without starting LR
Same result, by the time I'd cleaned all the crud with Healing Brush, launchd had written an extra 7GB or thereabouts. Hadn't even touch Camera Raw this time