The Feed-Cache of WordPress is in the database, it's in the table
options and sometimes you like to delete it. With a little SQL this is done quite quickly - but be careful, there is no way back! Within WordPress I do it with the help of the Plugin Adminer, which allows a complete mySQL access and you are still able to work in the backend of WordPress. Alternatively it's also possible with phpMyAdmin for example.
With the help of the following syntax you can delete the cache very fast, you have to adjust the example prefix
wp_ of course.
DELETE FROM `wp_options` WHERE `option_name` LIKE ('_transient%_feed_%')