This shows hardware faster.
Unchecking “Parallel processing” …
For simple projects, parallel provides no benefits. In other cases, if the source video format/encoding makes seeking difficult (e.g. B frames or long GOPs), when enabled it may cause slowdown as some frames are accessed out-of-order.
GPU usage still <2% though
Read the FAQ section about multiple cores. Understand the bottlenecks. The video does not originate in your GPU. Encode HEVC 3840x2160 and you will see it use more. See also Realtime (frame dropping)
Can somebody explain which value is recommended
It depends on what you plan to do with the exported video.