Trickplay 4k Image Generation not working

[TheDreadPirate]

To add to what gnattu said, enabling that setting doesn't reduce the number of images produced. It changes how ffmpeg reads and processes your media to create trickplays. Instead of creating images on frames at EXACTLY that time stamps set by the interval, it will only read key frames and produce trickplay images based on the nearest key frame. Key frames are "whole" images that don't require other frames to decode. How often key frames occur in your video depends on how it was encoded. If you DON'T enable this setting ffmpeg will need to start at a key frame and decode EVERY frame until it reaches the desired time stamp. Which could be a couple seconds worth of video or possibly much longer. Videos with only a key frame at the very beginning are a thing, though rare.

This will result in less "accurate" trick play images but MUCH faster trickplay creation. In 10.9 there was no "key frame" option for trickplays. IIRC, it too me 3 days to create trickplays for my library at the time. Now it completes in a couple hours when the key frame option was added. AND my library is much larger now.

You didn't specify your Synology model, but if it has a relatively recent Intel CPU you can enable some of the hardware acceleration options.