Before revision:  While this parallelization makes them computationally efficient, it restricts the model from fully exploiting the sequential nature of the input - the representation at a given layer can only access representations from lower layers, rather than the higher level representations already built in previous time steps .
After revision:  While this parallelization makes them computationally efficient, it restricts the model from fully exploiting the sequential nature of the input . The representation at a given layer can only access representations from lower layers, rather than the higher level representations already built in previous time steps .