I was having similar issues on an i3-6100/z170 and an i7-2675QM. Upgrading to a Ryzen 5 3600 / B450. Remedied my issues. I was having trouble getting more than 6m samples , immediately I was able to crank everything up. My i3-6100 was around 50% utilization as well. Software efficiency I’m sure is a big part too that’s why I’ve stuck to Ubuntu , try a Linux distribution (distro) to see if you get better results too .
I noticed stutters like your video in too. You can also reduce the bandwidth to see if that works. Multi-threaded software is difficult to write. IDK the program you are using (yet).