After upgrading from SQL 2005 to 2012, skipping 2005, users are experiencing random slowdowns. Where should I look for the problem?
Believe me that is not a simple question to answer.My first question would be
1. Did you updated stats for all databases after migration
2. Did you made sure indexes were all defragmented.
3. Did you tested your application on 2012.This you should have done.Look for slow performing queries
(select top 50
order by qs.total_worker_time desc) as highest_cpu_queries
cross apply sys.dm_exec_sql_text(plan_handle) as q
order by highest_cpu_queries.total_worker_time desc
When you find out queries which are taking long time .Look for there estimated query plan .That will tell you missing indexes or scans going if any.