Local rate limiter is slow to act

Networking
1

I tested the local rate limiter from istio docs. tuned it for 20 tokens/bucket , refill: 1sec : it is supposed to stop you at 20 req/sec .stressed my server (Jmeter) . rate limiter kicks after 3 seconds with 300 requests in total. not the best response but better than other settings I’ve tried . tried also 1200 tokens, refil every 60s . stressed the system: got error after 4000 requests.
why is the HTTP 429 response is fired so late ?
istio v1.11.3 but was observed in v1.9 too.

2 
apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: local-ratelimit-envoy-filter
  namespace: istio-system
spec:
  workloadSelector:
    labels:
      app: myapp
  configPatches:
    - applyTo: HTTP_FILTER
      match:
        context: SIDECAR_INBOUND
        listener:
          filterChain:
            filter:
              name: "envoy.filters.network.http_connection_manager"
      patch:
        operation: INSERT_BEFORE
        value:
          name: envoy.filters.http.local_ratelimit
          typed_config:
            "@type": type.googleapis.com/udpa.type.v1.TypedStruct
            type_url: type.googleapis.com/envoy.extensions.filters.http.local_ratelimit.v3.LocalRateLimit
            value:
              stat_prefix: http_local_rate_limiter
    - applyTo: HTTP_ROUTE
      match:
        context: SIDECAR_INBOUND
        routeConfiguration:
          vhost:
            name: "inbound|http|8080"
            route:
              action: ANY
      patch:
        operation: MERGE
        value:
          typed_per_filter_config:
            envoy.filters.http.local_ratelimit:
              "@type": type.googleapis.com/udpa.type.v1.TypedStruct
              type_url: type.googleapis.com/envoy.extensions.filters.http.local_ratelimit.v3.LocalRateLimit
              value:
                stat_prefix: http_local_rate_limiter
                token_bucket:
                  max_tokens: 20  # this will give us 20 req/sec
                  tokens_per_fill: 20
                  fill_interval: 1s
                filter_enabled:
                  runtime_key: local_rate_limit_enabled
                  default_value:
                    numerator: 100
                    denominator: HUNDRED
                filter_enforced:
                  runtime_key: local_rate_limit_enforced
                  default_value:
                    numerator: 100
                    denominator: HUNDRED
                response_headers_to_add:
                 - append: false
                   header:
                     key: Retry-After
                     value: '2'