

My first thought was similar - there might be some hardware acceleration happening for the jpgs that isn’t for the other formats, resulting in a CPU bottleneck. A modern harddrive over USB3.0 should be capable of hundreds of megabits to several gigabits per second. It seems unlikely that’s your bottleneck (though you can feel free to share stats and correct the assumption if this is incorrect - if your pngs are in the 40 megabyte range, your 3.5 per second would be pretty taxing).
If you are seeing only 1 CPU core at 100%, perhaps you could split the video clip, and process multiple clips in parallel?
I’ve had the same problem with HeliBoard learning garbage. I just changed my settings though, and I think it should help:
If you scroll all the way to the top again, you can manually manage the personal dictionary, including adding words you do want, and deleting any junk that was added by mistake, before switching that setting off.