Timeout error after SQL Server upgrade from 2008R2 to SQL Server 2014
The Cardinality Estimation logic was updated for SQL Server 2014 and could potentially be a reason. You would have to test the same queries with the old cardinality estimator and compare performance metrics. You could do this by lowering the compatibility level to &lt;120. 

I would perform all this testing on a test server and not in production.

https://msdn.microsoft.com/en-us/library/dn600374(v=sql.120).aspx
https://www.brentozar.com/archive/2014/04/sql-2014-cardinality-estimator-eats-bad-tsql-breakfast/