SQL Queries Slow in the Upgraded environment
Queries in need of query and index tuning are often those that are most vulnerable to performance regression after a SQL Server upgrade. That may be the best long term solution. It won't hurt to update stats to make sure the optimizer has the best information for informed choices.

If you identify issues where the legacy cardinality estimator performs significantly better after tuning, I suggest the most granular approach to address performance regressions. With a limited number of specific queries, use the FORCE_LEGACY_CARDINALITY_ESTIMATION hint. For a large number of queries within specific database(s), use ALTER DATABASE SCOPED CONFIGURATION. In the case where you experience CE problems with most or all databases, turn on trace flag 9481 at the instance level.

You might first try the ENABLE_QUERY_OPTIMIZER_HOTFIXES query hint or  QUERY_OPTIMIZER_HOTFIXES database scoped configuration so that patches made the CE available in the SP/CUs you've installed are used. These are not used by default due to an overabundance of caution.