Dal sito inglese mi hanno risposto così e sembra funzionare, vi tengo aggiornati!
The problem is caused by non-optimal database indexes. We have fixed this for the Joomfish final release.
You can add the indexes running the following queries:
ALTER TABLE `#__jf_content` ADD INDEX `combo` ( `reference_id` , `reference_field` , `reference_table` );
ALTER TABLE `#__jf_content` ADD INDEX `jfContent` ( `language_id` , `reference_id` , `reference_table` );
ALTER TABLE `#__jf_content` ADD INDEX `jfContentLanguage` (`reference_id`, `reference_field`, `reference_table`, `language_id`);