@vdw thanks for the hint and the links. Yes, reordering sequences so they have the same length in each batch is a nice idea. But actually I am wondering what is the effect of padding, packing and unpacking on the performance of the network, e.g. I mean not the computational performance but the loss performance. Why does padding result in a bad performance? By the way the project I am talking about can be found here (deeplio) 25. I would be thankful for any link or paper handling this topic!