Timeout error after SQL Server upgrade from 2008R2 to SQL Server 2014 We had a SQL Server 2008R2 Enterprise edition for our database to support a front-end application. We never had any timeout issues before. Recently the company decided to upgrade the database in to SQL Server 2014 Enterprise edition with 2 node always on cluster setup. The new server has better CPU, Memory than the old server.

After the upgrade I made all the necessary modifications, Checking Database consistency, run DBCC UPDATEUSAGE, update Statistics, index rebuilding, recompiling the stored procedures and so on. The database switching and migration all went well. However, our users start complaining about timeout issues. 

I have been reviewing different articles,blog posts and made some modification like changing the connection string and add MultiSubnetFailover = 'True', which seems helped a lot and minimize the frequency of timeout but still the issue is there.  Does anyone know what can cause this issue and how to resolve it?  I would highly appreciate your suggestions and recommendation to resolve this problem.
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/