I have a sluggishness in the WP admin and would like suggestions on how to identify and fix the problem. Our institution (USC) has been running a WP (3.3.1) sub-domain multisite system for over a year now. Recently we have been noticing issues with sluggishness in the admin. There are problems in a few sites, but for this post I only want to focus on our largest site. Various functions within that site are slow, and I would like to focus on the dashboard. This is our observation:
When the dashboard has not been access for a few hours, the next load takes over 20 seconds. Dashboard re-loads for the next hour or so are fast, <1 second.
Some site statistics because I believe the number of objects are affecting performance:
13,000 media objects
This is what I have done so far:
1) disable all plugins. No effect, dashboard still slow
2) installed a debugging plugin - debug objects (http://wordpress.org/extend/plugins/debug-objects/). I wanted to check certain things, especially database queries. According to the plugin, database queries took .5 seconds of the 20+second dashboard loadtime.
3) Verified php/WP memory. 128MB max, according to debug object only using 25% of the memory.
4) restarted httpd server. That should wipe out any memory cache which would make the next dashboard load take a long time. Dashboard load was fast which tells me memory or memory cache is probably not the problem.
So why is the dashboard taking so long to load? My guess is some type of information is being processed and probably stored back into WP (like a database cache). That is the only situation that I think of to explain our situation where the dashboard initially takes a long time to load, then is fast for the next hour or so.
Any suggestions on what I should next? This is what I will try next:
1) I have asked our sys admins to install a PHP profiler on our system: xdebug and xhprof. For further timing information.
2) export problem blog and import to a brand new WP install
3) upgrade to 3.3.2 to see if problem clears