When there is a lot of dense subtitle content, such as ASS subtitles converted from live bullet screen, the CPU usage rate is only 20%, and the encoding speed is very slow, probably only at a speed of 1x. However, when there are no subtitles or the subtitles are not dense, the speed can reach 5x and the CPU is fully loaded at 100%.
Using h264 or AMF has the same problem, and enable the OpenCL still the same.
ShanaEncoder version: 5.3.1.1
OS: windows 10 professional 22H2
CPU: AMD Ryzen 5800X
GPU: AMD 5700XT
There is discussion bulletin for non-korean speakers.
https://shana.pe.kr/shanaencoder_board_intl
Hardburn with ASS_sub needs more process in general.
Occasionally some instruction causes idle time that makes low CPU usage
I had similar problem few years ago in Handbrake. My CPU was Intel though...
Have you tried same encoding on ffmpeg?? You should test it for better answer :)