Acrylian,
I have done further research.
I did a clean install of Zenphoto version 1.4.14 [f5b47da52f] (Official build).
Using that version I can generate cache images at lightning speed on multiple albums.
Cache manager in 1.4.14 is capable of the following:
On 1 album, 120 images, 8 cache sizes, 960 images generated, less than 60 seconds total time.
Using mass-edit function: 7 subalbums, 297 images, 5 caches sizes, 1485 cache images generated, less than 60 seconds total time.
So using vers. 1.4.14 locally I could generate all required cache sizes for my entire gallery in a relatively short time, compress the Cache file and upload to server and everything is fine.
Vers. 1.5 & 1.5.1 Cache manager seems to be incapable of any of this. For one Cache manager no longer works and the mass-edit capabilities also seem to have been degraded (or not available at all, not sure about that bc cache manager hangs during processing of first album.)
So after introducing a code change that rendered 1000's of thumbnails useless, now you have degraded all capabilities to solve this problem that were previously available in 1.4.14. I've let it run for hours and it never starts to process again. PHP memory is set at 2048m.
Sorry to be negative. I am trying to help diagnose this problem, but I can only test, I don't read or write code.