Comment 7 for bug 1576067

Michael, the approach we (me and Andrey Grebennikov) took to see what things cause the most lags was the following one:

* in case of Horizon HA ensure that there is only one node working (to simplify further debugging)
* ensure that 'DEBUG' level of logging is set for all LOGGING['loggers'] related to various python-clients in /etc/openstack-dashboard/local_settings.py
* ensure also that whichever handler that is actually used for writing horizon logs (the ones defined within LOGGING['handlers']) has 'DEBUG' level enabled as well.
* preferrably truncate horizon logs to start fresh
* load the page you're experiencing difficulties with
* among all the logged calls that have been made while rendering the page, find the ones that took most time.

Parsing Horizon logs to see which calls take which time is actually the most tedious part of the process. In late N/early O (actually it was early O, but can be easily backported to the tagged N) release Horizon received a new 'OpenStack Profiler' panel which allows to do the same task from Horizon itself - reload the page with profiler switched on and then see the which calls were responsible for the rendering delays through a nice graphs.