How to check top Oracle wait classes since instance startup

Cheapest possible wait profile: sum time_waited per wait class since instance startup, average by number of waits. Whatever dominates here is where the instance has been spending time, and where tuning effort pays off most.

SELECT wait_class,
       SUM(time_waited) AS time_waited,
       SUM(time_waited) / SUM(total_waits) AS avg_wait
FROM   v$system_wait_class
GROUP BY wait_class
ORDER BY 3 DESC;