SQL Server 2012 doesn't use enough memory
Be sure that memory is the bottleneck

Here is a query that will tell you for every session, how much memory is being used versus the ideal amount of memory the optimiser would like to assign to the query. If the latter is higher than the former, then memory is a (but not necessarily the only) issue for you.

SELECT  s.session_id
        , r.granted_query_memory * 8 AS memory_used_kb
        , a.ideal_memory_kb
FROM    sys.dm_exec_sessions AS s
INNER JOIN sys.dm_exec_requests AS r ON r.session_id = s.session_id
LEFT JOIN sys.dm_exec_query_memory_grants a ON a.session_id = s.session_id