Computation time of bidirectional RNNs

Hi everyone,
According to Andrew Ng’s Bidirectional RNN:


The computation of backward directional of Bidirectional RNNs are calculated simultaneously with the computation of the forward direction - (Time complexity of O(n) for the sequence length) or the backward directional is only computed after the computation of the forward direction - (Time complexity of doubled O(n) for sequence length * num_direction)?

Thank you in advance!