Why is loading a field value from a group of nodes taking 30 seconds plus?
I'm guessing that the difference between the fast runs and the slow runs is caching. Within field_attach_load is a call to cache_get_multiple(). The field values can load from a cache which should be a lot faster: https://api.drupal.org/api/drupal/modules!field!field.attach.inc/function/field_attach_load/7

To investigate further I recommend using devel module's query logger. It can show the amount of time spent within queries. If that is not sufficient you can use a profiler like XHProf or Blackfire to see exactly which function calls are taking the most time.