What is the big oh complexity of Keras' GlobalMaxPooling1D() layer? More generally, how would you find the complexity of pooling layers?
It's a maximum of an unsorted array.
You can't know the maximum of an (unsorted) array without looking at all the elements of the array, and you can compute the maximum of the array after looking at each element exactly one time.
So the time complexity is linear in the number of activations being pooled.