*** UPDATE ***

In June Microsoft filed the above problem as a bug with MEMORY MANAGER, as they were able to reproduce the issue in their lab. Their development team is working on a fix. The only solution at this time is to reduce MAX MEMORY.

I am opening another case with Microsoft related to a similar issue. On our SQL 2012 EE servers with 256GB of memory, DBCC CHECKDB operations perform considerably slower with MAX MEMORY set to 230GB vs MAX MEMORY set to a lower number such as 46GB. We also see performance hits with UPDATE STATISTICS and are in the process of checking REINDEXING, which on the surface appears to experience similar delays due to large amounts of memory. I'll create a different post for these issues because I'm curious if anyone else has noticed the same behavior.