PHP 5.5/Wordpress running slow
I've got an Ubuntu 14.04 PHP 5.5 Wordpress 3.92 installation (using PHP5-FPM and Nginx).
Everything was going really well, until one of the clients created a very complex form (using Gravity Forms). He tested it on his personal dev server (Apache with PHP 5.4 and a local Mysql database) and the page loaded in about 2s - a bit slow but acceptable.
Then we copied it over to the DO server and the page refuses to load in less than 30s!
There is a load of spare RAM and originally the Mysql server was sat on a separate box, so to rule out network latency, I installed a db server locally - this has cut the time down to about 20s but it's still way too high.
Looking at top, everything seems to be CPU bound - but I don't understand why the (crappy) dev server would be OK and this production server isn't. Opcache is on by the way.
Any ideas on what else I should try?
PS: I've installed a cache plugin, but unfortunately, most users will be logged in so I can't rely on page-level caching (as there is user-specific stuff in the page header).