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 at specific intervals. You should type date and SNAP_ID
select snap_id,begin_interval_time,end_interval_time from dba_hist_snapshot where to_char(begin_interval_time,'DD-MON-YYYY')='20-FEB-2019' and EXTRACT(HOUR FROM begin_interval_time) between 8 and 10; select * from ( select active_session_history.event, sum(active_session_history.wait_time + active_session_history.time_waited) ttl_wait_time from dba_hist_active_sess_history active_session_history where event is not null and SNAP_ID between 34411 and 34431 group by active_session_history.event order by 2 desc) where rownum<10;
Query result is like below screenshot.