I got a time out error when creating a new category. My development has just over 10k courses.

The URL was http://localhost/moodle/dev/master/course/editcategory.php?parent=0

The category was actually created. Problem may have been in fixing the sort order but thats a guess.

Timed out while waiting for session lock.
Wait for your current requests to finish and try again later.
Stack trace:

line 1237 of /lib/dml/mysqli_native_moodle_database.php: dml_sessionwait_exception thrown
line 593 of /lib/sessionlib.php: call to mysqli_native_moodle_database->get_session_lock()
line ? of unknownfile: call to database_session->handler_read()
line 184 of /lib/sessionlib.php: call to session_start()
line 473 of /lib/sessionlib.php: call to session_stub->__construct()
line 60 of /lib/sessionlib.php: call to database_session->__construct()
line 692 of /lib/setup.php: call to session_get_instance()
line 29 of /config.php: call to require_once()
line 9 of /course/editcategory.php: call to require_once()