Understand and Analyse AWR Report

The Automatic Workload Repository (AWR) collects, processes, and maintains performance statistics for problem detection and self-tuning purposes. 
Compare the AWR report for the period with bad performance with that of a one taken when performance was good, also a lot depends on type of problem you have, your system, your application, time of day etc.


SNAPSHOTS:

Creating Snapshots From Grid Control(automatically generated every hour), go to:

DB instance-Server tab-Automatic Workload Repository-Run AWR report- By Snapshot(Begin & End)

Creating Snapshots Manually:

You can manually create snapshots with the CREATE_SNAPSHOT procedure if you want to capture statistics at times different than those of the automatically generated snapshots. For example:

BEGIN
  DBMS_WORKLOAD_REPOSITORY.CREATE_SNAPSHOT ();
END;
/

Begin with:
1) Top 5 Timed Foreground Events
2) Operating System Statistics -- how much busy and wait ...
3) SQL statistics -- like top SQL's
4) Instance Activity 
5) I/O stats.
6) look out at advisors (memory-sga,pga, Undo & so on)
7) Buffer waits
8) Instance Efficiency Percentages (Target 100%)

Also check for SQL Statistics:

• SQL ordered by Elapsed Time
• SQL ordered by CPU Time
• SQL ordered by User I/O Wait Time
• SQL ordered by Gets
• SQL ordered by Reads
• SQL ordered by Physical Reads (UnOptimized)
• SQL ordered by Executions
• SQL ordered by Parse Calls
• SQL ordered by Sharable Memory
• SQL ordered by Version Count
• Complete List of SQL Text

Reference:
NOTE:1086120.1 - Quick Instructions For Obtaining The Automatic Workload Repository (AWR) Report
FAQ: How to Use AWR reports to Diagnose Database Performance Issues [ID 1359094.1]
Using AWR/Statspack reports to help solve some Portal Performance Problems scenarios [ID 565812.1]