Why the first prediction of neural network in PyTorch is slower than following predictions?

So I have ResNet50 trained to classify images.

For each prediction I track the time needed for it (input and model are moved to GPU):

    start = time.time()
    result = self.model.forward(transformed_image)
    end = time.time()
    print(end - start)

And always I get the following output:

1.0592937469482422
0.05996203422546387
0.06096029281616211
0.04996800422668457

So the first prediction is ~20 times longer than the following ones.

Why? And what happens behind the scenes when we launch prediction for the first time, using Torch?