High Query Wait Times
That looks like a snippet from the definition of the sys.sp_spaceused system stored procedure.


  regularly having very high wait stats


What are the waits pertaining to the execution of this stored procedure?

I'd be even more interested to find out why sys.sp_spaceused is running so frequently that it is negatively affecting performance.  That stored procedure isn't typically going to be run regularly, or frequently if regularly.

Depending on what version of SQL Server you are using, there are a few ways to find out who (host name, application name, etc.) is kicking this stored procedure off.  My favorite way to do this would be through the use of Extended Events (if you are only SQL Server 2008 or higher.  You haven't specified version in your question as of now).