Queries are performing bad after SQL Server 2008 SP4 upgrade
We opened a support ticket with Microsoft and they gave us a tool to run to gather metrics when issue was in progress. I couldn't run the tool during business hours since I didn't want it cause problems for working processes. Then in the evening, update statistics job ran for almost twice long than before. Probably that produced better query plans for queries running on next morning i.e. on Tuesday. It was way better than Monday. I observed the performance today. It's back to normal. I didn't have to use Microsoft tool at all. Go figure! Thank you.