diff --git a/Sources/SwiftWarningControl/WarningControlRegions.swift b/Sources/SwiftWarningControl/WarningControlRegions.swift
index 1d0b9abea87..c9a108f4014 100644
--- a/Sources/SwiftWarningControl/WarningControlRegions.swift
+++ b/Sources/SwiftWarningControl/WarningControlRegions.swift
@@ -130,22 +130,35 @@ public struct WarningControlRegionTree {
     guard !controls.isEmpty else { return }
     let newNode = WarningControlRegionNode(range: range)
     for (diagnosticGroupIdentifier, control) in controls {
-      // Handle the control for the added diagnostic group
-      // and propagate it to all of its subgroups.
+      // Breadth-first walk over the inheritance tree of `diagnosticGroupIdentifier`,
+      // applying `control` to every transitive sub-group.
+      //
+      // We use an index-based queue (rather than `Array.removeFirst()`, which is O(N))
+      // and dedupe at dequeue time. Previously, a sub-group reachable from more than one
+      // already-queued group would be enqueued multiple times and have its control
+      // re-applied each time, which was both redundant and made the loop O(N^2) in the
+      // queue size. For a diamond-shaped inheritance graph this caused the queue to
+      // grow exponentially, so the function would not terminate in any reasonable time.
       var groups: [DiagnosticGroupIdentifier] = [diagnosticGroupIdentifier]
+      var groupIndex = 0
       var processedGroups: Set<DiagnosticGroupIdentifier> = []
-      while !groups.isEmpty {
-        let groupIdentifier = groups.removeFirst()
-        processedGroups.insert(groupIdentifier)
+      while groupIndex < groups.count {
+        let groupIdentifier = groups[groupIndex]
+        groupIndex += 1
+        // Skip groups we already reached via another path through the inheritance tree.
+        guard processedGroups.insert(groupIdentifier).inserted else { continue }
+
         newNode.addWarningGroupControl(for: groupIdentifier, control: control)
         if control != .ignored {
           enabledGroups.insert(diagnosticGroupIdentifier)
         }
 
-        let newSubGroups = groupInheritanceTree.subgroups(of: groupIdentifier).filter { !processedGroups.contains($0) }
-        // Ensure we add a corresponding control to each direct and
-        // transitive sub-group of the one specified on this control.
-        groups.append(contentsOf: newSubGroups)
+        // Enqueue not-yet-processed sub-groups so each direct and transitive sub-group
+        // also receives the control.
+        for subGroup in groupInheritanceTree.subgroups(of: groupIdentifier)
+        where !processedGroups.contains(subGroup) {
+          groups.append(subGroup)
+        }
       }
     }
     insertIntoSubtree(newNode, parent: rootRegionNode)
diff --git a/Tests/PerformanceTest/WarningControlRegionsPerformanceTests.swift b/Tests/PerformanceTest/WarningControlRegionsPerformanceTests.swift
new file mode 100644
index 00000000000..b73b79c3336
--- /dev/null
+++ b/Tests/PerformanceTest/WarningControlRegionsPerformanceTests.swift
@@ -0,0 +1,124 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftIfConfig
+import SwiftParser
+import SwiftSyntax
+import SwiftWarningControl
+import XCTest
+import _InstructionCounter
+import _SwiftSyntaxTestSupport
+
+/// Linear inheritance chain: g0 -> g1 -> g2 -> ... -> gN.
+/// Exercises the BFS but the queue never exceeds 1 element (each level has 1 sub-group).
+private func makeChain(depth: Int) -> DiagnosticGroupInheritanceTree {
+  var subGroups: [DiagnosticGroupIdentifier: [DiagnosticGroupIdentifier]] = [:]
+  for i in 0..<depth {
+    subGroups[DiagnosticGroupIdentifier("g\(i)")] = [DiagnosticGroupIdentifier("g\(i + 1)")]
+  }
+  return try! DiagnosticGroupInheritanceTree(subGroups: subGroups)
+}
+
+/// Wide-and-shallow tree: one root with N direct sub-groups.
+/// Exercises `Array.removeFirst()` shift cost on a large queue.
+private func makeFan(width: Int) -> DiagnosticGroupInheritanceTree {
+  var children: [DiagnosticGroupIdentifier] = []
+  for i in 0..<width {
+    children.append(DiagnosticGroupIdentifier("g\(i)"))
+  }
+  return try! DiagnosticGroupInheritanceTree(
+    subGroups: [DiagnosticGroupIdentifier("root"): children]
+  )
+}
+
+/// Diamond DAG: each level has 2 nodes; both point to both nodes at the next level.
+/// Exercises de-duplication. Without de-dup at dequeue, the queue grows as 2^depth,
+/// so even small depths blow up.
+private func makeDiamondDAG(depth: Int) -> DiagnosticGroupInheritanceTree {
+  var subGroups: [DiagnosticGroupIdentifier: [DiagnosticGroupIdentifier]] = [:]
+  func id(_ level: Int, _ side: Int) -> DiagnosticGroupIdentifier {
+    DiagnosticGroupIdentifier("l\(level)s\(side)")
+  }
+  for level in 0..<depth - 1 {
+    subGroups[id(level, 0)] = [id(level + 1, 0), id(level + 1, 1)]
+    subGroups[id(level, 1)] = [id(level + 1, 0), id(level + 1, 1)]
+  }
+  return try! DiagnosticGroupInheritanceTree(subGroups: subGroups)
+}
+
+final class WarningControlRegionsPerformanceTests: XCTestCase {
+
+  /// Calls the public entry point with `globalControls` so we exercise
+  /// `addWarningGroupControls` (which contains the BFS of interest).
+  private func buildTree(seed: String, inheritance: DiagnosticGroupInheritanceTree) {
+    let parsed = Parser.parse(source: "")
+    _ = parsed.warningGroupControlRegionTree(
+      configuredRegions: .empty,
+      globalControls: [(DiagnosticGroupIdentifier(seed), .error)],
+      groupInheritanceTree: inheritance
+    )
+  }
+
+  private func runAndReport(
+    label: String,
+    inheritance: DiagnosticGroupInheritanceTree,
+    seed: String,
+    iterations: Int
+  ) {
+    for _ in 0..<3 {
+      buildTree(seed: seed, inheritance: inheritance)
+    }
+
+    let startInstructions = getInstructionsExecuted()
+    let t0 = Date()
+    for _ in 0..<iterations {
+      buildTree(seed: seed, inheritance: inheritance)
+    }
+    let wall = Date().timeIntervalSince(t0)
+    let endInstructions = getInstructionsExecuted()
+    let instructions = endInstructions - startInstructions
+
+    print(
+      "[BENCH] \(label) iters=\(iterations) wall=\(String(format: "%.3f", wall * 1000))ms "
+        + "instructions=\(instructions) per-call=\(instructions / UInt64(iterations))"
+    )
+  }
+
+  func testWarningControlChainBenchmark() throws {
+    try XCTSkipIf(longTestsDisabled)
+    print("--- chain (linear inheritance; queue stays tiny so this is mostly a baseline) ---")
+    for depth in [50, 200, 800] {
+      let inh = makeChain(depth: depth)
+      runAndReport(label: "chain depth=\(depth)", inheritance: inh, seed: "g0", iterations: 20)
+    }
+  }
+
+  func testWarningControlFanBenchmark() throws {
+    try XCTSkipIf(longTestsDisabled)
+    print("--- fan (root has N direct sub-groups; exercises removeFirst() shift cost) ---")
+    for width in [50, 200, 800, 1600] {
+      let inh = makeFan(width: width)
+      runAndReport(label: "fan width=\(width)", inheritance: inh, seed: "root", iterations: 20)
+    }
+  }
+
+  func testWarningControlDiamondBenchmark() throws {
+    try XCTSkipIf(longTestsDisabled)
+    // Note: keep depths small. Without the de-dup fix the queue grows as 2^depth so
+    // even depth 10 takes too long for a unit-test run.
+    print("--- diamond DAG (each level has 2 nodes pointing to both of next level) ---")
+    for depth in [4, 6, 8] {
+      let inh = makeDiamondDAG(depth: depth)
+      runAndReport(label: "diamond depth=\(depth)", inheritance: inh, seed: "l0s0", iterations: 10)
+    }
+  }
+}
diff --git a/Tests/SwiftWarningControlTest/WarningControlTests.swift b/Tests/SwiftWarningControlTest/WarningControlTests.swift
index 3c0db43b211..cd05ee3c7cc 100644
--- a/Tests/SwiftWarningControlTest/WarningControlTests.swift
+++ b/Tests/SwiftWarningControlTest/WarningControlTests.swift
@@ -346,6 +346,31 @@ public class WarningGroupControlTests: XCTestCase {
     )
   }
 
+  /// A sub-group that is reachable from multiple parents (a diamond-shaped
+  /// inheritance graph) must still have the control applied to it. Previously,
+  /// the BFS in `addWarningGroupControls` did not de-duplicate at dequeue, so
+  /// such a sub-group was enqueued (and re-processed) once per incoming path;
+  /// for deeper diamonds this caused the queue to grow exponentially.
+  func testDiamondSubGroupInheritance() throws {
+    try assertWarningGroupControl(
+      """
+      @diagnose(SuperGroup, as: error)
+      func foo() {
+        1️⃣let x = 1
+      }
+      """,
+      groupInheritanceTree: DiagnosticGroupInheritanceTree(subGroups: [
+        "SuperGroup": ["ChildA", "ChildB"],
+        "ChildA": ["SharedGroup"],
+        "ChildB": ["SharedGroup"],
+      ]),
+      diagnosticGroupID: "SharedGroup",
+      states: [
+        "1️⃣": .error
+      ]
+    )
+  }
+
   func testInheritanceTreeCycle() throws {
     XCTAssertThrowsError(
       try DiagnosticGroupInheritanceTree(subGroups: [