When you take many complaints from customer about slowness of database, you should check Oracle database wait events.
You can check Top Oracle database wait events in Cache which is v$ queries with below script.
select wait_class, sum(total_waits), sum(time_waited) from gv$session_wait_class where wait_class !='Idle' group by wait_class order by 3 desc;
You can check Top Oracle database wait events from Active session history which is v$active_session_history queries with below script.
select * from ( select active_session_history.event, sum(active_session_history.wait_time + active_session_history.time_waited) ttl_wait_time from gv$active_session_history active_session_history where active_session_history.event is not null group by active_session_history.event order by 2 desc) where rownum <= 10;
Query result is like below screenshot.