I want to share a problem I faced today to maybe help someone else 
while running the 04_mnist_basics I start feeling that that from the 13’s cell
the code running very slow but by looking at the
executed time below I saw the code running fast but something else slow it down
after some troubleshooting I found the problem:
It seems that the problem is that Variable Inspector given by nbextensions may slow down the notebook in some cases: Large variables cause varInspector to slow down entire notebook. · Issue #1275 · ipython-contrib/jupyter_contrib_nbextensions · GitHub
so by disabling it the problem solved