Symptoms
Threads are not scheduled as evenly in Microsoft SQL Server 2012 or SQL Server 2014 Standard Edition as they are in SQL Server 2012 or SQL Server 2014 Enterprise Edition. Therefore, performance in SQL Server 2012 or SQL Server 2014 Standard Edition is slower than expected when there are many concurrent requests.
Resolution
The issue was first fixed in the following cumulative update of SQL Server.
Cumulative Update 1 for SQL Server 2014 /en-us/help/2931693
Cumulative Update 7 for SQL Server 2012 SP1 /en-us/help/2894115
About cumulative updates for SQL Server
Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server:
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.