Reason for application is running slow from a DBA side in SQL

There might be some reasons behind it:
1. Blocking situation
2. lack of userfull indexes
3. Lack of updated statistics
4. deadlock situation
5. network issue

6.Check the status of query.It can be either of two reasons.Either external or internal factor.External factors are resource crunches .Internal factors are because of missing index, invalid cache plan, fragmentation,so on.Starting point to identify higher level and then do a deep dive into particular area.