You might check out these other threads.
A significant fix for those issues was included in 1.19.4.2935. 1.19.5.3006 even seemed to include more changes, but it was pulled back for other problems. Maybe the next version will help you; maybe it’s worth waiting for that.
You said this was the only process running - there aren’t any other Transcoder processes at all - playback, media optimizer, intro detection?
You could test with media you have stored locally, to rule out any network/NAS interaction. Not my first guess, but an easy thing to rule out.
Your system is fast where my system is slow. I noticed that running the Transcoder to extract these images, the same way Plex does, but excluding the “-progressurl” argument, was MUCH faster for me.
You could try something similar.
