Few metrics that indicate some general problem categories
TOTAL_CPU_TIME (Higher than normal) :: High processor usage is a good indicator that the slowdown is located within data server processing. The data server is working harder than normal to handle application requests.
CLIENT_IDLE_WAIT_TIME (Normal) :: The slowdown is NOT located in the client.
TOTAL_WAIT_TIME (Normal) :: Requests are NOT spending more time waiting for resources needed such as locks or for input and output.
TOTAL_SECTION_PROC_TIME (Higher than normal) :: The cost of SQL statements (access plan sections) has increased.