Installing production MongoDB cluster in GKE using the Bitnami Helm chart

Hi all,
We recently tried installing MongoDB in our GKE cluster using the Bitnami Helm chart. We deployed a replicaset with 4 replicas using the configuration below:

  debug: false
  existingSecret: mongodb-secret # Existing secret with MongoDB credentials (keys: mongodb-password, mongodb-root-password, mongodb-replica-set-key)
  rootUser: user
architecture: replicaset
replicaCount: 3
  enabled: true
  enabled: true

When switching from our current Atlas production cluster (M30) to the new self-hosted one, we experienced that the new cluster was very slow compared to the old one (a factor 20). What can we do to improve on this?
Things we talked about:
Use a premium storage class instead of default (an SSD)
Use designated node-pool instead of one shared with other pods
We have about 22k users minimum 40 active at the same time and maximum 500 - we want this to scale in the future. What can we do? Someone mentioned using an XFS disk? Other improvements to a production environment using the Bitnami Helm chart?