These options let you control the performance of thumbnail generation.
Override thumbnail threads: Turn this on to override the number of background threads Opus uses to generate thumbnails. As thumbnail generation is mostly a CPU intensive task (decoding JPEG images, for example), increasing the number of threads can greatly speed up the generation of thumbnails, although you may see diminishing returns as shared resources like storage become the bottleneck.
We don't recommend setting this higher than your CPU's thread count, and it's best to leave at least one thread free for other tasks.
When the Override thumbnail threads checkbox is turned off, Opus automatically chooses how many threads to use based on your CPU. This is this recommended mode, and means you do not need to adjust your configuration when moving between different computers.
These options let you control whether Opus caches thumbnails or not. Caching thumbnails takes up some disk space but can result in much quicker loading of thumbnails once they have been generated and cached. You can enable or disable caching for different types of drive (e.g. you might want to cache thumbnails over the network but not for local drives).