diff --git a/app/alarm/ui/src/main/java/org/phoebus/applications/alarm/ui/table/AlarmTableUI.java b/app/alarm/ui/src/main/java/org/phoebus/applications/alarm/ui/table/AlarmTableUI.java
index 04c268910d..fd9036a42e 100644
--- a/app/alarm/ui/src/main/java/org/phoebus/applications/alarm/ui/table/AlarmTableUI.java
+++ b/app/alarm/ui/src/main/java/org/phoebus/applications/alarm/ui/table/AlarmTableUI.java
@@ -695,24 +695,43 @@ private void limitAlarmCount(final List<AlarmInfoRow> alarms,
      */
     private void update(final ObservableList<AlarmInfoRow> items, final List<AlarmInfoRow> input)
     {
-        // Similar, but might trigger a full table redraw:
-        // items.setAll(input);
-
-        // Update content of common list entries
-        int N = Math.min(items.size(), input.size());
-        for (int i=0; i<N; ++i)
-            items.get(i).copy(input.get(i));
+        // The ObservableList was created with the CHANGING_PROPERTIES extractor, so every
+        // property set inside copy() fires a list-change event.  The SortedList wrapping
+        // it responds to each event by re-sorting, producing O(N²·log N) work for N rows.
+        //
+        // Fix: temporarily detach the SortedList's comparator before the copy loop.
+        // With a null comparator the SortedList mirrors the source in insertion order and
+        // skips the re-sort on item-property events.  A single O(N·log N) sort happens
+        // when we rebind at the end.
+        final TableView<AlarmInfoRow> table = (items == active_rows) ? active : acknowledged;
+        @SuppressWarnings("unchecked")
+        final SortedList<AlarmInfoRow> sorted = (SortedList<AlarmInfoRow>) table.getItems();
+        sorted.comparatorProperty().unbind();
+        sorted.setComparator(null);
+
+        try
+        {
+            // Update content of common list entries
+            int N = Math.min(items.size(), input.size());
+            for (int i=0; i<N; ++i)
+                items.get(i).copy(input.get(i));
 
-        N = input.size();
-        if (N > items.size())
+            N = input.size();
+            if (N > items.size())
+            {
+                // Additional elements if input is larger that existing list
+                for (int i=items.size(); i<N; ++i)
+                    items.add(input.get(i));
+            }
+            else // Trim items, input has fewer elements
+                items.remove(N, items.size());
+        }
+        finally
         {
-            // Additional elements if input is larger that existing list
-            for (int i=items.size(); i<N; ++i)
-                items.add(input.get(i));
+            // Rebind: SortedList immediately acquires the table comparator and
+            // performs exactly one sort of all N updated rows.
+            sorted.comparatorProperty().bind(table.comparatorProperty());
         }
-        else // Trim items, input has fewer elements
-            items.remove(N, items.size());
-
     }
 
     /** Select all rows that match the current 'search' pattern */
diff --git a/app/alarm/ui/src/test/java/org/phoebus/applications/alarm/AlarmTableUpdatePerformanceTest.java b/app/alarm/ui/src/test/java/org/phoebus/applications/alarm/AlarmTableUpdatePerformanceTest.java
new file mode 100644
index 0000000000..7728c5ca04
--- /dev/null
+++ b/app/alarm/ui/src/test/java/org/phoebus/applications/alarm/AlarmTableUpdatePerformanceTest.java
@@ -0,0 +1,207 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Canadian Light Source Inc.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *******************************************************************************/
+package org.phoebus.applications.alarm;
+
+import javafx.collections.FXCollections;
+import javafx.collections.ObservableList;
+import javafx.collections.transformation.SortedList;
+import org.junit.jupiter.api.Test;
+import org.phoebus.applications.alarm.client.AlarmClientLeaf;
+import org.phoebus.applications.alarm.client.ClientState;
+import org.phoebus.applications.alarm.model.SeverityLevel;
+import org.phoebus.applications.alarm.ui.table.AlarmInfoRow;
+
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.lessThan;
+
+/**
+ * Regression test for the O(N^2 log N) performance bug in AlarmTableUI.update().
+ *
+ * <p>The ObservableList for alarm rows is created with the CHANGING_PROPERTIES
+ * extractor.  A SortedList wraps it with a comparator.  Each call to
+ * AlarmInfoRow.copy() fires 8 property-change events, and each event causes the
+ * SortedList to perform a full O(N log N) sort.  For N rows this gives
+ * O(8N * N log N) = O(N^2 log N) total comparator work.
+ *
+ * <p>The fix: temporarily set the SortedList comparator to null before the copy
+ * loop so property-change events do not trigger re-sorts.  Restore the comparator
+ * once after the loop, producing exactly one O(N log N) sort.
+ *
+ * <p>These tests use only javafx.base classes (ObservableList, SortedList,
+ * SimpleStringProperty, ...) which work headlessly without starting the JavaFX
+ * application toolkit.
+ *
+ * @see <a href="https://github.com/ControlSystemStudio/phoebus/issues/3504">Issue #3504</a>
+ */
+@SuppressWarnings("nls")
+class AlarmTableUpdatePerformanceTest
+{
+    private static final int N = 150;  // Large enough to expose O(N^2) but fast to run
+
+    /** Build a list of N AlarmInfoRow items and a corresponding list of N "update" rows. */
+    private static AlarmInfoRow makeRow(final int index, final SeverityLevel sev)
+    {
+        // AlarmClientLeaf(parent_path, name) works standalone – no tree needed.
+        final AlarmClientLeaf leaf = new AlarmClientLeaf("/Test/Area", "pv" + String.format("%04d", index));
+        leaf.setState(new ClientState(sev, "msg", String.valueOf(index),
+                Instant.now(), sev, "msg"));
+        return new AlarmInfoRow(leaf);
+    }
+
+    /**
+     * Demonstrate the O(N^2 log N) problem: with the CHANGING_PROPERTIES extractor
+     * active and a comparator present, each of the 8 * N property-change events fired
+     * by the copy loop triggers a full re-sort of the SortedList.
+     */
+    @Test
+    void testOldBehaviorTriggersExcessiveSorts()
+    {
+        final ObservableList<AlarmInfoRow> rows =
+                FXCollections.observableArrayList(AlarmInfoRow.CHANGING_PROPERTIES);
+
+        final List<AlarmInfoRow> inputs = new ArrayList<>(N);
+        for (int i = 0; i < N; i++)
+        {
+            rows.add(makeRow(i, SeverityLevel.MAJOR));          // "existing" row
+            inputs.add(makeRow(N - 1 - i, SeverityLevel.MINOR)); // "update" row (reversed name)
+        }
+
+        final AtomicInteger comparisons = new AtomicInteger();
+        final Comparator<AlarmInfoRow> countingComparator =
+                (a, b) -> { comparisons.incrementAndGet(); return a.pv.get().compareTo(b.pv.get()); };
+
+        final SortedList<AlarmInfoRow> sorted = new SortedList<>(rows, countingComparator);
+
+        // Reset after the initial sort that SortedList performs on construction.
+        comparisons.set(0);
+
+        // OLD: copy with comparator active — each property change fires a re-sort.
+        for (int i = 0; i < N; i++)
+            rows.get(i).copy(inputs.get(i));
+
+        final int oldComparisons = comparisons.get();
+
+        // One sort of N elements costs at most N*log2(N) comparisons.
+        final int singleSortUpperBound = N * 20; // generous upper bound for 1 sort
+        System.out.println("Old comparisons: " + oldComparisons + "  (one-sort bound: " + singleSortUpperBound + ")");
+
+        // The old code triggers far more comparisons than a single sort would.
+        assertThat("Old code should trigger many more comparisons than a single sort",
+                oldComparisons, greaterThan(singleSortUpperBound));
+        // Confirm sorted view has all N rows (exercises the sorted variable).
+        assertThat(sorted.size(), equalTo(N));
+    }
+
+    /**
+     * Validate the fix: suppressing the comparator during the copy loop and
+     * restoring it once reduces total comparator calls to a single O(N log N) sort.
+     * The final row order must also be correct.
+     */
+    @Test
+    void testFixSuppressesIntermediateSortsAndPreservesOrder()
+    {
+        final ObservableList<AlarmInfoRow> rows =
+                FXCollections.observableArrayList(AlarmInfoRow.CHANGING_PROPERTIES);
+
+        final List<AlarmInfoRow> inputs = new ArrayList<>(N);
+        for (int i = 0; i < N; i++)
+        {
+            rows.add(makeRow(i, SeverityLevel.MAJOR));
+            inputs.add(makeRow(N - 1 - i, SeverityLevel.MINOR));
+        }
+
+        final AtomicInteger comparisons = new AtomicInteger();
+        final Comparator<AlarmInfoRow> countingComparator =
+                (a, b) -> { comparisons.incrementAndGet(); return a.pv.get().compareTo(b.pv.get()); };
+
+        final SortedList<AlarmInfoRow> sorted = new SortedList<>(rows, countingComparator);
+        comparisons.set(0); // reset after initial sort
+
+        // FIX: suppress the comparator, copy all rows, restore comparator once.
+        sorted.setComparator(null);
+        for (int i = 0; i < N; i++)
+            rows.get(i).copy(inputs.get(i));
+        sorted.setComparator(countingComparator); // triggers exactly one O(N log N) sort
+
+        final int fixedComparisons = comparisons.get();
+
+        // Only one sort should have happened.
+        final int singleSortUpperBound = N * 20;
+        System.out.println("Fixed comparisons: " + fixedComparisons + "  (one-sort bound: " + singleSortUpperBound + ")");
+
+        assertThat("Fix should produce at most one sort's worth of comparisons",
+                fixedComparisons, lessThan(singleSortUpperBound));
+
+        // Final order must be ascending by PV name.
+        assertThat(sorted.size(), equalTo(N));
+        for (int i = 0; i < N - 1; i++)
+        {
+            final String a = sorted.get(i).pv.get();
+            final String b = sorted.get(i + 1).pv.get();
+            assertThat("Sorted list must be in ascending PV name order at index " + i,
+                    a.compareTo(b), lessThan(1));
+        }
+    }
+
+    /**
+     * End-to-end comparison: run both the old and fixed code paths on identical input
+     * and verify that the fix uses at least 5x fewer comparator calls while producing
+     * the same final sort order.
+     */
+    @Test
+    void testFixOutperformsOldCodeAndProducesSameOrder()
+    {
+        final List<AlarmInfoRow> inputs = new ArrayList<>(N);
+        for (int i = 0; i < N; i++)
+            inputs.add(makeRow(N - 1 - i, SeverityLevel.MINOR));
+
+        // --- Old path ---------------------------------------------------
+        final AtomicInteger oldCount = new AtomicInteger();
+        final Comparator<AlarmInfoRow> oldCmp =
+                (a, b) -> { oldCount.incrementAndGet(); return a.pv.get().compareTo(b.pv.get()); };
+        final ObservableList<AlarmInfoRow> oldRows =
+                FXCollections.observableArrayList(AlarmInfoRow.CHANGING_PROPERTIES);
+        for (int i = 0; i < N; i++) oldRows.add(makeRow(i, SeverityLevel.MAJOR));
+        final SortedList<AlarmInfoRow> oldSorted = new SortedList<>(oldRows, oldCmp);
+        oldCount.set(0);
+        for (int i = 0; i < N; i++) oldRows.get(i).copy(inputs.get(i));
+
+        // --- Fixed path -------------------------------------------------
+        final AtomicInteger newCount = new AtomicInteger();
+        final Comparator<AlarmInfoRow> newCmp =
+                (a, b) -> { newCount.incrementAndGet(); return a.pv.get().compareTo(b.pv.get()); };
+        final ObservableList<AlarmInfoRow> newRows =
+                FXCollections.observableArrayList(AlarmInfoRow.CHANGING_PROPERTIES);
+        for (int i = 0; i < N; i++) newRows.add(makeRow(i, SeverityLevel.MAJOR));
+        final SortedList<AlarmInfoRow> newSorted = new SortedList<>(newRows, newCmp);
+        newCount.set(0);
+        newSorted.setComparator(null);
+        for (int i = 0; i < N; i++) newRows.get(i).copy(inputs.get(i));
+        newSorted.setComparator(newCmp);
+
+        System.out.println("Comparison counts — old: " + oldCount.get() + "  fixed: " + newCount.get()
+                + "  ratio: " + (oldCount.get() / Math.max(1, newCount.get())) + "x");
+
+        assertThat("Fix must use at least 5x fewer comparator calls",
+                oldCount.get(), greaterThan(newCount.get() * 5));
+
+        // Both paths must produce the same sorted order.
+        assertThat(oldSorted.size(), equalTo(newSorted.size()));
+        for (int i = 0; i < N; i++)
+            assertThat("Row " + i + " must match between old and fixed paths",
+                    oldSorted.get(i).pv.get(), equalTo(newSorted.get(i).pv.get()));
+    }
+}