The index.yaml will regenerate every request to /index.yaml even though there is no any package changed.

When we compare the LastModified between objects from cache and storage:
https://github.com/helm/chartmuseum/blob/master/pkg/storage/storage.go#L64

The objects from cache is out of order due to map ranging....
https://github.com/helm/chartmuseum/blob/master/pkg/chartmuseum/server/multitenant/index.go#L107

func (server *MultiTenantServer) getRepoObjectSlice(entry *cacheEntry) []cm_storage.Object {
	var objects []cm_storage.Object
	for _, entry := range entry.RepoIndex.Entries {
		for _, chartVersion := range entry {
			object := cm_repo.StorageObjectFromChartVersion(chartVersion)
			objects = append(objects, object)
		}
	}
	return objects
}

And the LastModifed timestamp of object from storage seems has higher precision (I only test it on s3 backend)

LastModifed from cache: 2018-06-11T03:31:52.000Z
LastModifed from storage: 2018-06-11T03:31:52.094Z

Those above would cause the regenerating of index.yaml every request.