While working on one upgrade project, after successfully completion of all activities there were multiple performance issues experienced. While we were trying to troubleshoot the issue, may be in wrong direction; found this great article today from Sudhir Nory which explains the root cause. This may not be the issue for all performance issue on upgrade projects but is surly helpful to know.
When a database is upgraded to SQL Server 2014 from any earlier version of SQL Server, the database retains its existing compatibility level if it is at least 100(SQL Server 2008 R2). If the compatibility-level setting is 110 or lower it uses the old query optimizer which may not be effective. We found the current compatibility level displayed in the Compatibility level list box was SQL server 2008 for the ORG_MSCRM Database SQL Server 2014 includes substantial improvements to the component that creates and optimized query plans and gets utilized only when database compatibility level is reset to 120 (SQL server 2014).