My proposal:

Dolphin already supports "Remember display style for each folder". I suggest we make thumbnail previews to be preserved on a per folder basis when "Remember display style for each folder" is enabled.

My reasoning:
The current implementation of generating thumbnails has two disadvantages:
1) Thumbnail previews for every video (even when it's not needed) just wastes resources (processing power and disk space).
2) Thumbnail previews generated indiscriminately for all folders when a user needs them only for a specific folder creates visual clutter. That is especially true in folders where the user has selected the Compact View with very small icons. Thumbnails in these circumstances offer little, so the user should be given the option to pick and choose WHERE they want to have thumbnails.