"openstack server list" takes 30s

Hi, I built a Ussuri cluster with 3 controllers and 5 compute nodes. OpenStack CLI ran pretty fast at the beginning, but gets slower over time along with increased workloads. Right now, it takes about 30s to list 10 VMs. The CPU, memory and disk usage are on those 3 controllers are all in the range. I understand there are many API calls happening behind CLI. I'd like to figure out how this 30s is consumed, which call is the killer. Any guidance or hint would be helpful. Thanks! Tony