After upgrade from D6 to D7, workflow uses all my CPU and slows down the site extremely

I just upgraded my D6 site to D7 including the workflow module. Except for one error (see #2265147: After upgrade there is some error: Column not found: 1054 Unknown column 'base.name' in 'field list') the upgrade seems to work. But creating a table view with either "Workflow current: New state name" or "Workflow states (deprecated): Current state name" slows down my site extremely and makes mysqld use almost all my CPU.

In mysql, SHOW FULL PROCESSLIST shows me some queries that won't disappear until I restart the mysql server. They look like this:

     Id: 850
   User: [USER]
   Host: localhost
     db: [DB]
Command: Query
   Time: 932
  State: Sending data
   Info: SELECT COUNT(*) AS expression
FROM 
(SELECT 1 AS expression
FROM 
node node
LEFT JOIN workflow_node workflow_node ON node.nid = workflow_node.nid
LEFT JOIN workflow_states workflow_states ON workflow_node.sid = workflow_states.sid
LEFT JOIN workflow_node_history workflow_node_current ON ( SELECT max(hid) FROM workflow_node_history
                            where nid = node.nid
                            and   sid != old_sid ) = workflow_node_current.hid
WHERE (( (node.status = '1') ))) subquery