That will consume almost all processing power if it's CPU-only and is a very slow process.

This is a complicated topic and the terminology is a bit ambiguous.

Yes, non-hardware-accelerated transcoding is slow and will consume the CPU.

However, you don't necessarily need an external GPU to do hardware-accelerated transcoding. When you use Intel QuickSync for example, the codec hardware is part if the CPU. On the other hand it is only in CPUs that have integrated graphics, so you could still say the transcoding is done "by the GPU", just not the additional one that you put in. In fact, putting in a dedicated graphics card often disables the integrated graphics and you have to use tricks to re-enable it before you can use it for transcoding again.