Well, I think I found a way to pretty much max out all 6 cores. I was trying an app for saving videos on the net and it can save up to 4k and convert to an mp4 from whatever the original is, if the video is at 4k quality to begin with.
My first tests were usually 1080p, but I'm doing a 7 minute 4k video right now and noticed all 6 cores are often at 90-95%! But as I continue my other work at the same time, things seem to be working fine. It is taking a lot of time though (1080p files were fast). It seems the conversion is the slow part.