Just to check, how long does it take to create thumbnails for a movie? There are a few reasons this can become very slow even on fast systems. If it’s more than a few minutes per movie, something might be wrong.
This is doable but I’m not aware of anything already assembled. Using the GPU wouldn’t dramatically accelerate the process.
When Plex is extracting thumbnails you can see the command that it’s executing in the process table and in the logs. An ffmpeg equivalent is something like this:
ffmpeg -skip_frame:v nokey -i input.mkv -q 3 -filter_complex "[0:V:0] fps=fps=0.500000:round=up,scale=w=320:h=240:force_original_aspect_ratio=decrease [out]" -map "[out]" img-%06d.jpg
Then they’re assembled together into index-sd.bif files. There are a few ways to do this. Relevant search keywords that may help are index-sd.bif, Roku and BIF.
Then you’d need to put the created index-sd.bif file in the correct location for Plex. Here’s a recent discussion about that.
