Operating system: windows 10 1908
Graphics card: Dual GTX 980 + 5700xt
CPU: Threadripper 3960x
RAM: 64gb 3400mhz
When rendering with the CPU only I can render a frame in 15 min, When using the gpus (both gtx 980) alone they take about an hour when using the CPU and GPU together it takes about 35 min, which is faster than the GPU's alone but slower than the CPU alone.
At a guess this is due to running CUDA on the CPU.