Description of problem: In performance testing of acm 2.15 rbac ui, when creating a role assignment, after selecting the clusters in the table view and when choosing the namespace, there is a latency of around 30 seconds on average for an env that has 500 managed clusters with 500 namespace each. This is because search is leverage to fetch the shared namespaces

Steps to Reproduce:

1. install acm 2.15 and toggle the fine grain rbac to true
2. leverage search to populate a cluster with fake managedcluster data with 500 managedclusters that have 500 namespaces on it each
3. attempt to create a role assignment and go to the table ui
4. select two clusters have common shared namespaces
5. click on the namespaces and notice it takes around 30 seconds to populate the dropdown

Additional info: This is a performance issue that might not be a bug but it scales as larger the number gets, creating this Jira ticket to see what the approach would be as well as see if there could be a optimization made if needed to increase performance

Also in my findings, i found that this performance issue happens in having the number of clusters and namespaces in the environment, the amount of vms didnt matter in each namespace