Back in the days when GPU mining was a thing, CPU mining was reasonably efficient. This was actually a design goal, it was seen as promoting decentralization.

Back in the days when GPU mining was a thing, CPU mining was reasonably efficient. This was actually a design goal, it was seen as promoting decentralization. Now, obviously a rig full of Vega cards put out a lot higher hash rate than an 8C CPU, but the CPU was actually reasonably efficient in terms of wattage. At the time, building quad-CPU rigs on older architectures was actually a reasonably efficient build.