Improve extremely slow 'drupal_process_form'
Based on the backtrace, you won't know why your site is slow.

You should run a proper PHP profiler and analyse the results which can identify the slowest components.

It doesn't have to be Drupal it-self, but could be lack of caching or misconfiguration.

Some suggestions:


disable modules which you don't need (e.g. Devel, core Database Logging module, etc.),
upgrade your PHP to the latest or higher branch,
recompile your PHP for 64-bit or higher architecture depending on your CPU,
use the faster storage device for your db files or whole LAMP environment (e.g. SSD or memory-based filesystem),
use PHP debugger or profiler to find out any performance bottleneck (e.g. XDebug Profiler, DTrace or NuSphere PhpED PHP Profiler),
run some time-consuming drush command under gprof profiling tool, so you can find some performance bottleneck as well.


If disabling slow modules or profiling won't help, see: How do you improve Drupal performance?