"resmgr:cpu quantum' means we are deliberately throttling some sessions to avoid exceeding a CPU resource limit.

This might be overall server load being too much for the machine, or it might be artificially constrained by some setting, eg if you set cpu_count to 10, then after 10 cpu's "worth" of work, we will start to limit things. (Some material here https://www.oracle.com/technetwork/database/performance/instance-caging-wp-166854.pdf )

Start with your standard AWR reports. There is rule that says it is the sessions that are having this wait that are the issue - they might be just the victims of other sessions being too hard on the server.

Check your standard metrics - "DB Time", "Average Active Sessions" etc.

Also, check your alert log to ensure that you are not errantly in a resource manage plan that you did not intend to have active. (Note - there are some preconfigured ones even if you didnt create any). You could explore *temporarily* turning off resource manager (set the parameter to null) to see if it has an impact to help with root cause detection.