Your model training (or inference) is too slow, or takes too much memory? You should profile where the computing time or memory is spend. This is less specific about RETURNN but more about TensorFlow, so please refer to the TensorFlow documentation for more recent details.

In RETURNN, there is the option store_metadata_mod_step which has the effect that every Nth step, it will do the with these additional options:

run_metadata = tf.RunMetadata()

That will be written to the TF event file, so you can see additional information about runtime and memory usage in TensorBoard. Also, it will write a timeline in Google Chrome trace format (visit chrome://tracing in Chrome and open that trace file).

See also this for further information: