When using F5 BIG-IP load balancer of GKE On-Prem you might be wanting to use evaluation license. Keep in mind that this license has a restriction of 2MBps bandwidth in total. GKE-OP even with one user cluster can cause saturation and slowness of K8s API response. With multiple cluster and Istio installed the API can stop response at all. Note that F5 might not be showing the bandwith is saturated when you use the CLI tools.
Resolution: use full license or request 10GBps evaluation license.