The phpbb use the session_id as primary key for php_sessions table. Value of session_id in phpbb is random. The mysql innodb storage internally store the data sorted by primary key. After add new record to the sessions table innodb fully rewrite this table in new order (because the new session_id not at the end of table). On highload forums this make the overhead mysql server.
To resolve this problem I'm change the session_id to unique key and created the fake id autoincrement field to use it as primary key.