There are few settings that can't be changed by the user interface. These parameters are defined with PHP constants.
To override them, rename the file config.default.php to config.php.
Actually, the following constants can be overrided:
<?php
// HTTP_TIMEOUT => default value is 20 seconds (Maximum time to fetch a feed)
define('HTTP_TIMEOUT', '20');
// HTTP_MAX_RESPONSE_SIZE => Maximum accepted size of the response body in MB (default 2MB)
defined('HTTP_MAX_RESPONSE_SIZE') or define('HTTP_MAX_RESPONSE_SIZE', 2097152);
// DATA_DIRECTORY => default is data (writable directory)
define('DATA_DIRECTORY', __DIR__.'/data');
// DB_FILENAME => default value is db.sqlite (default database filename)
define('DB_FILENAME', 'db.sqlite');
// ENABLE_MULTIPLE_DB => default value is true (multiple users support)
define('ENABLE_MULTIPLE_DB', true);
// DEBUG_FILENAME => default is data/debug.log
define('DEBUG_FILENAME', DATA_DIRECTORY.'/debug.log');
// THEME_DIRECTORY => default is themes
define('THEME_DIRECTORY', 'themes');
// SESSION_SAVE_PATH => default is empty (used to store session files in a custom directory)
define('SESSION_SAVE_PATH', '');
// PROXY_HOSTNAME => default is empty (make HTTP requests through a HTTP proxy if set)
define('PROXY_HOSTNAME', '');
// PROXY_PORT => default is 3128 (default port of Squid)
define('PROXY_PORT', 3128);
// PROXY_USERNAME => default is empty (set the proxy username is needed)
define('PROXY_USERNAME', '');
// PROXY_PASSWORD => default is empty
define('PROXY_PASSWORD', '');
// ENABLE_AUTO_UPDATE => default is true (enable Miniflux update from the user interface)
define('ENABLE_AUTO_UPDATE', true);
// SUBSCRIPTION_CONCURRENT_REQUESTS => number of concurrent feeds to refresh at once
// Reduce this number on systems with limited processing power
define('SUBSCRIPTION_CONCURRENT_REQUESTS', 5);