Many images do not open in Icon Preview Mode w/ Sierra or Mavericks, ok w/ Snow Leopard

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Image folders on a 1TB USB3 external containing +/-25,000 images for 450gb.
8GB folder with +/-100 images does not open ALL the previews, stops trying after 4 minutes. Images in this folder range from 10-270mb.
Opening a folder that is 3GB w/ +/-100 images is fine in icon view, takes perhaps less than a minute and all open.

This issue is present with:
MacMini 2012 - 2.5gHz, 8GB Ram, Sierra
MBP 2012 - 2.5gHz, 16gb, Mavericks

This issue is NOT present with:
Mini 2010 2.5gHz c2d 5gb Ram, Snow Leopard
Large 8gb folder - all previews open, +/- 2 min.


All three machines have a HDD.

Why do the newer machines w/ a newer OS fail to open many of the previews?

 

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Just tried a MBA w/ SSD (Sierra) and it seems no better. Perhaps if the image files were on an SSD as well but would have concerns about fragmentation as these files are much larger than the 20mb size that Apple automatically defragments.

Shall have to jsut spend a bunch of time splitting up the large folders to such size the the Sierra find can handle.