Is it safe to truncate a field revision table?
I think there the Field SQL norevisions module will help you out:


  Field SQL norevisions provides a field storage backend that prevents
  entity revisions from being saved to the MySQL storage backend.
  
  The purpose of the module is to reduce the amount duplicated data that
  is saved on large Drupal sites that do not use revisions. It has the
  additional performance benefit of halving the number of queries during
  creating entities with fields.


This module will not allow revision fields to be created so it will increase your performance as number of queries will get reduced, besides that you will have no revisions on nodes further.

Also you can use some custom module codes provided on following URL:

http://www.ambidev.com/make-your-drupal-7-faster-by-removing-all-revisions/

and this module to delete revision entries:

https://www.drupal.org/project/node_revision_delete