I've recently being doing some bulk import/export/delete using the /civicrm/import/contact for import/export /civicrm/contact/search/advanced for delete
I've done some timings for the bulk delete action and see the following (on a system with 2GB RAM) Advanced Search: Show as contributions
Select contributions, bulk action delete 100 contribution delete: 28secs 200 contribution delete: 48secs 400 contribution delete: 88secs 1000 contribution delete: ~208secs 42% ram
memory usage increase by ~0.2% per second on 2GB system.
You will note that to delete 1000 contributions it will use nearly 1GB RAM (we've got php memory limits turned right up for testing).
Looking in jira I've across https://issues.civicrm.org/jira/browse/CRM-19798 which looks like it may be similar but not sure if it is related.
CiviCRM is version 4.7.14 on drupal 7.52.