doesn't use CPU fully

I have a top-of-the-line Mac Pro(late 2013, macOS Sierra, 12core, 64GB, 1TB SSD, Firepro D700)

It works fine on the PC, but when I export it using Premiere or Media Encoder on my Mac Pro, the CPU usage is only 20 ~ 30%.

I'd appreciate it if you could tell me how to fix it.