GPU. DAZ Studio uses Iray which needs an Nvidia card. If a scene can't fit the card's VRAM, it will fallback to CPU rendering which is slow as a molasses.
As watdapakisdis said, but VRAM isn't everything, CUDA CORES are the important part of a GPU to improve rendering time on DAZ, the more the better, you can even get 2 GPUs to increase the number of CUDA CORES and potentially get double the performance.
Actually, both are correct and slightly wrong at the same time, you need to match your CPU with the CUDA CORES with your GPU. a faster GPU will be waiting for instructions from a slower CPU, thus slowing production down and vise versa, a fast CPU will be waiting for a slower GPU to finish it's last instructions.