Jupter notebook very slow

I want to share a problem I faced today to maybe help someone else :slightly_smiling_face:

while running the 04_mnist_basics I start feeling that that from the 13โ€™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. ยท Issue #1275 ยท ipython-contrib/jupyter_contrib_nbextensions ยท GitHub

so by disabling it the problem solved