On a big site with more then 20 000 nodes, when creating particular node I first experienced WSOD, then as I reported errors, max_time_execution passed, then when this was raised memory limit passed. So I raised both to high limits, memory to 1gb and maxe_execution time to forever. Then I run the node/add and what I have to see is
Page execution time was 30257.61 ms. Memory used at: devel_boot()=2.21 MB, devel_shutdown()=707.38 MB, PHP peak=711.75
What really happend is that this content type had a entity reference field in it, so when I tried to add or edit it, it had to make a "select list" for over 10 000 nodes, which took so much memory and time. So this was an easy fix, I used autocomplete form and all is solved.
What was the problem is also no error or info on this was shown by drupal, would be good that select widget has a limit and makes some warning or error, as this way you dont get it so easy that problem lies with widget.