Multi-card training

Hello, I want to use two cards (RTX 6000) for training, and the following are my training instructions. nvidia-smi also shows that both cards are working, but the first round of training takes a long time to complete. Why is that?
`python train.py --gpu_ids 0,1 --batch_size 4 --num_threads 8 --dataroot ./datasets/maps --name maps_cyclegan --model cycle_gan`