diff --git a/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporter.java b/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporter.java
index 6361f0edc6b5..8ad7f1d1164a 100644
--- a/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporter.java
+++ b/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporter.java
@@ -18,7 +18,23 @@
 
 public interface PrometheusExporter {
 
+    /**
+     * Update the Prometheus metrics in text format.
+     *
+     * NOTE: capacity data is refreshed independently by {@code AlertManagerImpl}'s own
+     * periodic {@code CapacityChecker} timer. Do NOT force a synchronous
+     * {@code recalculateCapacity()} call here: it spins up a fresh thread pool per host
+     * and per storage pool across ALL zones on every single scrape, so with Z zones a
+     * single Prometheus scrape triggered Z redundant full recalculations. That extra,
+     * uncoordinated load compounds over time and can lead to {@code scrape_duration_seconds}
+     * climbing until a management-server restart.
+     *
+     * @see PrometheusExporterImpl#updateMetrics()
+     */
     void updateMetrics();
 
+    /**
+     * @return the latest Prometheus metrics refreshed by {@link #updateMetrics()}.
+     */
     String getMetrics();
 }
diff --git a/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporterImpl.java b/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporterImpl.java
index b49f11c77745..f737bad25ca2 100644
--- a/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporterImpl.java
+++ b/plugins/integrations/prometheus/src/main/java/org/apache/cloudstack/metrics/PrometheusExporterImpl.java
@@ -32,7 +32,6 @@
 import org.apache.cloudstack.storage.datastore.db.ImageStoreDao;
 import org.apache.commons.lang3.StringUtils;
 
-import com.cloud.alert.AlertManager;
 import com.cloud.api.ApiDBUtils;
 import com.cloud.api.query.dao.DomainJoinDao;
 import com.cloud.api.query.dao.StoragePoolJoinDao;
@@ -126,8 +125,6 @@ public String toString() {
     @Inject
     private DomainJoinDao domainDao;
     @Inject
-    private AlertManager alertManager;
-    @Inject
     DedicatedResourceDao _dedicatedDao;
     @Inject
     private AccountDao _accountDao;
@@ -497,7 +494,6 @@ public void updateMetrics() {
             for (final DataCenterVO dc : dcDao.listAll()) {
                 final String zoneName = dc.getName();
                 final String zoneUuid = dc.getUuid();
-                alertManager.recalculateCapacity();
                 addHostMetrics(latestMetricsItems, dc.getId(), zoneName, zoneUuid);
                 addVMMetrics(latestMetricsItems, dc.getId(), zoneName, zoneUuid);
                 addVolumeMetrics(latestMetricsItems, dc.getId(), zoneName, zoneUuid);
diff --git a/server/src/main/java/com/cloud/alert/AlertManagerImpl.java b/server/src/main/java/com/cloud/alert/AlertManagerImpl.java
index 7bf00037ee4b..c85b3aa0117a 100644
--- a/server/src/main/java/com/cloud/alert/AlertManagerImpl.java
+++ b/server/src/main/java/com/cloud/alert/AlertManagerImpl.java
@@ -161,6 +161,8 @@ public class AlertManagerImpl extends ManagerBase implements AlertManager, Confi
 
     private final ExecutorService _executor;
 
+    private ExecutorService _capacityExecutorService;
+
     protected SMTPMailSender mailSender;
     protected String[] recipients = null;
     protected String senderAddress = null;
@@ -249,6 +251,9 @@ public boolean start() {
     @Override
     public boolean stop() {
         _timer.cancel();
+        if (_capacityExecutorService != null) {
+            _capacityExecutorService.shutdown();
+        }
         return true;
     }
 
@@ -281,6 +286,24 @@ public void sendAlert(AlertType alertType, long dataCenterId, Long podId, String
         }
     }
 
+    /**
+     * Shared, long-lived pool for capacity recalculation, reused across every
+     * recalculateHostCapacities()/recalculateStorageCapacities() call instead of creating and
+     * tearing down a new thread pool per invocation. Repeatedly creating/shutting down pools was
+     * unnecessary overhead under frequent callers (e.g. the Prometheus exporter used to trigger a
+     * full recalculation on every scrape, see https://github.com/apache/cloudstack/issues/13586).
+     * Lazily created so this remains safe for callers that invoke the recalculate methods directly
+     * without going through configure()/start() (e.g. unit tests).
+     */
+    private synchronized ExecutorService getCapacityExecutorService() {
+        if (_capacityExecutorService == null || _capacityExecutorService.isShutdown()) {
+            _capacityExecutorService = Executors.newFixedThreadPool(
+                    Math.max(1, CapacityManager.CapacityCalculateWorkers.value()),
+                    new NamedThreadFactory("Capacity-Calculator"));
+        }
+        return _capacityExecutorService;
+    }
+
     /**
      * Recalculates the capacities of hosts, including CPU and RAM.
      */
@@ -290,10 +313,8 @@ protected void recalculateHostCapacities() {
             return;
         }
         ConcurrentHashMap<Long, Future<Void>> futures = new ConcurrentHashMap<>();
-        ExecutorService executorService = Executors.newFixedThreadPool(Math.max(1,
-                Math.min(CapacityManager.CapacityCalculateWorkers.value(), hostIds.size())));
         for (Long hostId : hostIds) {
-            futures.put(hostId, executorService.submit(() -> {
+            futures.put(hostId, getCapacityExecutorService().submit(() -> {
                 final HostVO host = hostDao.findById(hostId);
                 _capacityMgr.updateCapacityForHost(host);
                 return null;
@@ -307,7 +328,6 @@ protected void recalculateHostCapacities() {
                         entry.getKey(), e.getMessage()), e);
             }
         }
-        executorService.shutdown();
     }
 
     protected void recalculateStorageCapacities() {
@@ -316,10 +336,8 @@ protected void recalculateStorageCapacities() {
             return;
         }
         ConcurrentHashMap<Long, Future<Void>> futures = new ConcurrentHashMap<>();
-        ExecutorService executorService = Executors.newFixedThreadPool(Math.max(1,
-                Math.min(CapacityManager.CapacityCalculateWorkers.value(), storagePoolIds.size())));
         for (Long poolId: storagePoolIds) {
-            futures.put(poolId, executorService.submit(() -> {
+            futures.put(poolId, getCapacityExecutorService().submit(() -> {
                 Transaction.execute(new TransactionCallbackNoReturn() {
                     @Override
                     public void doInTransactionWithoutResult(TransactionStatus status) {
@@ -343,7 +361,6 @@ public void doInTransactionWithoutResult(TransactionStatus status) {
                         entry.getKey(), e.getMessage()), e);
             }
         }
-        executorService.shutdown();
     }
 
     @Override