diff --git a/litellm/integrations/prometheus_services.py b/litellm/integrations/prometheus_services.py
index a5f2f0b5c72..52b32c12764 100644
--- a/litellm/integrations/prometheus_services.py
+++ b/litellm/integrations/prometheus_services.py
@@ -105,6 +105,13 @@ class PrometheusServicesLogger:
         return metrics
 
     def is_metric_registered(self, metric_name) -> bool:
+        # Use registry's collector dict to avoid REGISTRY.collect(), which is O(all metrics)
+        # and causes severe perf regression when a new Router (and thus this logger) is
+        # created per request (e.g. hierarchical router_settings from DB).
+        names_to_collectors = getattr(self.REGISTRY, "_names_to_collectors", None)
+        if names_to_collectors is not None:
+            return metric_name in names_to_collectors
+        # Fallback for custom registries that don't expose _names_to_collectors
         for metric in self.REGISTRY.collect():
             if metric_name == metric.name:
                 return True
diff --git a/tests/test_litellm/integrations/test_prometheus_services.py b/tests/test_litellm/integrations/test_prometheus_services.py
index b627d31fda0..3b2b806084b 100644
--- a/tests/test_litellm/integrations/test_prometheus_services.py
+++ b/tests/test_litellm/integrations/test_prometheus_services.py
@@ -1,6 +1,7 @@
 import json
 import os
 import sys
+import time
 from unittest.mock import AsyncMock, patch
 
 import pytest
@@ -17,6 +18,79 @@ sys.path.insert(
 )  # Adds the parent directory to the system path
 
 
+def test_is_metric_registered_does_not_use_registry_collect():
+    """
+    Validates the perf regression fix: is_metric_registered() must NOT call REGISTRY.collect()
+    when _names_to_collectors is available. collect() is O(all metrics) and causes severe
+    slowdown when a new Router (and thus PrometheusServicesLogger) is created per request
+    (e.g. hierarchical router_settings from DB). See GitHub issue #19921.
+    """
+    from prometheus_client import CollectorRegistry, Counter, Histogram
+
+    # Simulate a proxy with many metrics (e.g. router, redis, postgres, llm providers)
+    registry = CollectorRegistry()
+    for i in range(80):
+        Counter(
+            f"litellm_service_{i}_total_requests",
+            "Total requests",
+            labelnames=["service"],
+            registry=registry,
+        )
+        Histogram(
+            f"litellm_service_{i}_latency",
+            "Latency",
+            labelnames=["service"],
+            registry=registry,
+        )
+
+    pl = PrometheusServicesLogger()
+    pl.REGISTRY = registry
+
+    # Mock collect() - the bug is that is_metric_registered() calls it (expensive)
+    original_collect = registry.collect
+    collect_called = []
+
+    def track_collect(*args, **kwargs):
+        collect_called.append(1)
+        return original_collect(*args, **kwargs)
+
+    registry.collect = track_collect
+
+    # Simulate Router init: many is_metric_registered() calls; time to show fix vs no-fix difference
+    n_calls = 30 * 2  # 30 iters × 2 names
+    start = time.perf_counter()
+    for _ in range(30):
+        pl.is_metric_registered("litellm_service_0_latency")
+        pl.is_metric_registered("litellm_service_79_total_requests")
+    elapsed_s = time.perf_counter() - start
+    elapsed_ms = elapsed_s * 1000
+    per_call_us = (elapsed_s / n_calls) * 1_000_000 if n_calls else 0
+    n_collect = len(collect_called)
+
+    # Clear latency output (visible with pytest -s)
+    path = "slow (REGISTRY.collect)" if n_collect else "fast (_names_to_collectors)"
+    print(
+        f"\n  is_metric_registered latency: {elapsed_ms:.2f} ms total | "
+        f"{per_call_us:.1f} µs/call | {n_calls} calls | {n_collect} collect() invocations | {path}\n"
+    )
+
+    # With fix: _names_to_collectors is used, collect() must not be called, elapsed ~ms
+    # With fix commented out: collect() every time → slow (e.g. 50–500ms+), test fails
+    assert n_collect == 0, (
+        f"is_metric_registered() must not use REGISTRY.collect() when _names_to_collectors "
+        f"is available (perf regression).\n"
+        f"  Latency: {elapsed_ms:.2f} ms total, {per_call_us:.1f} µs per call, {n_calls} calls.\n"
+        f"  collect() was called {n_collect} times (slow path).\n"
+        f"  With fix: typically <1 ms total, 0 collect() calls. "
+        "Uncomment the _names_to_collectors branch in prometheus_services.is_metric_registered()"
+    )
+    # With fix, assert we're in the fast path (optional; keeps regression visible)
+    assert elapsed_s < 0.05, (
+        f"is_metric_registered() took {elapsed_ms:.2f} ms for {n_calls} calls "
+        f"({per_call_us:.1f} µs/call); expected <50 ms with _names_to_collectors."
+    )
+
+
 def test_create_gauge_new():
     """Test creating a new gauge"""
     pl = PrometheusServicesLogger()