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ActiveUsersPager generates a query like this when a user name is supplied as a starting value:

SELECT /* IndexPager::buildQueryInfo (ActiveUsersPager) */ rc_user_text AS user_name, rc_user_text, MAX(rc_user) AS user_id, COUNT(*) AS recentedits FROM `recentchanges` FORCE INDEX (rc_user_text) WHERE (rc_user > 0) AND (rc_log_type IS NULL OR rc_log_type != 'newusers') AND (rc_timestamp >= '?') AND (NOT EXISTS (SELECT 1 FROM `ipblocks` WHERE (rc_user=ipb_user) AND ipb_deleted = '1' )) AND (rc_user_text >= '?') GROUP BY rc_user_text ORDER BY rc_user_text LIMIT 101;

Without the rc_user_text >= '?' clause the query executes in seconds. With the clause it takes minutes. Removing the FORCE INDEX doesn't help.

EXPLAIN from an enwiki slave:

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: recentchanges
         type: range
possible_keys: rc_user_text
          key: rc_user_text
      key_len: 273
          ref: NULL
         rows: 4013039  <-- eek!
        Extra: Using index condition; Using where
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: ipblocks
         type: ref
possible_keys: ipb_user
          key: ipb_user
      key_len: 4
          ref: enwiki.recentchanges.rc_user
         rows: 1
        Extra: Using where
2 rows in set (0.25 sec)

The functionality to specify a starting rc_user_text value should be disabled, or the page redesigned.