From the Microsoft KB article:
“After you upgrade to Microsoft SQL Server 2014, you notice a decrease in performance in Microsoft Team Foundation Server (TFS) 2013 Update 2.”
“These issues occur because the hardware requirements for SQL Server 2014 are greater than those for ealier versions of SQL Server. “
if you don’t upgrade Compatibility level to SQL 2014 you should not be affected by the performance as the new CE is only used with Compatibility level 2014
As you can read in the referenced KB article:
“To specifically work around the Work Item Tracking performance issue, use the following method:
If you see a significant decrease in performance when you open or edit work items after you upgrade to SQL Server 2014, change the Compatibility Level setting of your databases to 110. This causes SQL Server to use the earlier cardinality estimator tool. This tool more accurately estimates the number of rows that are returned by the joins.”
–Neno