The table:

SHOW CREATE TABLE wp_user2role2object_rs

Its creation:

CREATE TABLE `wp_user2role2object_rs` (
 `assignment_id` bigint(20) unsigned NOT NULL auto_increment,
 `user_id` bigint(20) unsigned default NULL,
 `group_id` bigint(20) unsigned default NULL,
 `role_name` varchar(64) NOT NULL,
 `role_type` enum('rs','wp','wp_cap') NOT NULL default 'rs',
 `scope` enum('blog','term','object') NOT NULL,
 `src_or_tx_name` varchar(32) NOT NULL default '',
 `obj_or_term_id` bigint(20) unsigned NOT NULL default '0',
 `assign_for` enum('entity','children','both') NOT NULL default 'entity',
 `inherited_from` bigint(20) unsigned NOT NULL default '0',
 `assigner_id` bigint(20) unsigned NOT NULL default '0',
 `date_limited` tinyint(2) NOT NULL default '0',
 `start_date_gmt` datetime NOT NULL default '0000-00-00 00:00:00',
 `end_date_gmt` datetime NOT NULL default '2035-01-01 00:00:00',
 `content_date_limited` tinyint(2) NOT NULL default '0',
 `content_min_date_gmt` datetime NOT NULL default '0000-00-00 00:00:00',
 `content_max_date_gmt` datetime NOT NULL default '2035-01-01 00:00:00',
 PRIMARY KEY  (`assignment_id`),
 KEY `role2obj` (`scope`,`assign_for`,`role_type`,`role_name`,`src_or_tx_name`,`obj_or_term_id`),
 KEY `role2agent` (`assign_for`,`scope`,`role_type`,`role_name`,`group_id`,`user_id`),
 KEY `role_rs` (`date_limited`,`role_type`,`role_name`,`scope`,`assign_for`,`src_or_tx_name`,`group_id`,`user_id`,`obj_or_term_id`),
 KEY `role_assignments` (`role_name`,`role_type`,`scope`,`assign_for`,`src_or_tx_name`,`group_id`,`user_id`,`obj_or_term_id`,`inherited_from`,`assignment_id`),
 KEY `content_date_limited` (`content_date_limited`)
) ENGINE=MyISAM AUTO_INCREMENT=7911 DEFAULT CHARSET=utf8

In the slow query log:

# Time: 121112 22:22:46
# User@Host: cscircles[cscircles] @ localhost []
# Query_time: 0  Lock_time: 0  Rows_sent: 0  Rows_examined: 7892
SELECT uro.role_name, uro.content_date_limited, uro.content_min_date_gmt, uro.content_max_date_gmt  FROM wp_user2role2object_rs AS uro WHERE uro.scope = 'blog' AND uro.role_type = 'rs'   AND ( uro.group_id = '5' OR uro.user_id = '7512' );

Explaining the query:

EXPLAIN SELECT uro.role_name, uro.content_date_limited, uro.content_min_date_gmt, uro.content_max_date_gmt
FROM wp_user2role2object_rs AS uro
WHERE uro.scope =  'blog'
AND uro.role_type =  'rs'
AND (
uro.group_id =  '5'
OR uro.user_id =  '7512'

Its explanation

id: 1, select_type: SIMPLE, table: uro, type: ALL, possible_keys: role2obj, key: NULL, key_len: NULL, ref: NULL, rows: 5919, Extra: Using where