If your drive is the bottleneck, this will make things worse. If you want to proceed:

You’re already using ffmpeg to get the sequence of frames, correct? You can add the -ss and -t flags to give a start time and a duration. Generate a list of offsets by dividing the length of video by the number of processes you want, and feed them through gnu parallel to your ffmpeg command.

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?

Considering that you are not using their software, was the laptop worth the premium you paid for it, vs buying from Clevo directly?

I figured the hardware and software coming from the same vendor would yield the best results, and wanted to support a company that supports right-to-repair, and Linux in general. But ultimately I found Pop!_OS buggy and had performance issues, so I’m not using their OS, and their firmware is causing issues with my SSD, so I’d like to be off of it as well (but was told "there’s no process for reverting to the proprietary firmware“ for the specific model I have). I could have bought a Clevo directly, saving hundreds of dollars, and probably had a better working machine.

Would not recommend System76. I’ve had many issues with my machine (primarily software, related to their buggy custom firmware, and Pop!_OS, until I ditched that for stock Ubuntu). Their support has been terrible - rather similar to OP’s, actually. I’ve had the laptop for about 2.5 years, and I’m checking practically daily for something to replace it.