Couple of updates...
Further investigating this issue I found that indeed ffmpeg was creating thumbnails forever, given the qnap processing power isnt that huge.
Some mitigation was possible by specifying a smaller resolution size of the created thumbs, but it was still too heavy of a load for me.
So I decided to migrate from the TS-212 to a TS-253A, and what can I say, hell of a difference.
The problem however is that on the TS-212 (x19 ARM) and the TS-253A (x86_64) the behaviour of Twonky Server seems different:
While on the TS-212 ffmpeg was always running and creating thumbs, on the TS-253A ffmpeg is not starting at all. As a result, now, on the TS-253A no thumbs are showing up when checking in the webUI, unless artwork is embedded into the media container (e.g. an MP4 file).
I suspect that the startup-scripts of each platform package are taking different decisions, but I am not certain how to find out.
Anyone out there who can help, pretty please?