When I upgraded by database to the newest version, performance is now much slower. Why?
Most likely, you need to update the statistics of all the indexes in your new SQL Server database. The easiest way to do this is to create a database maintenance plan and select the “Update statistics used by query optimizer” from the “Update Data Optimization Information” screen of the maintenance wizard, then run it now, or schedule it to run whenever the SQL Server is not being heavily used. Updating statistics is a I/O intensive task, and it can slow down other SQL Server operations when it is running.