Describe the bug:
Increasing CPU usage.

Expected behaviour:
Even CPU usage.

Steps to reproduce the bug:
Start the pod, watch CPU load.

Anything else we need to know?:
We are experiencing ever increasing cpu load in the webhook component. I've attached a graph of about 2 weeks:

When the pod is restarted, the CPU load will start to rise again from a low value.

Environment details::

• Kubernetes version (e.g. v1.10.2): v1.16.3
• Cloud-provider/provisioner: Own / Kubeadm.
• cert-manager version: v0.12.0
• Install method: static manifests

/kind bug