diff --git a/Assets/Tests/EditMode/Simulation/BuildZoneOverlayQueryTests.cs b/Assets/Tests/EditMode/Simulation/BuildZoneOverlayQueryTests.cs
index 1a843ce..931e1a5 100644
--- a/Assets/Tests/EditMode/Simulation/BuildZoneOverlayQueryTests.cs
+++ b/Assets/Tests/EditMode/Simulation/BuildZoneOverlayQueryTests.cs
@@ -104,5 +104,57 @@ public void TheTwoReads_DistinguishOutsideFromInsideButSpacingBlocked()
                 Is.EqualTo(CommandResultCode.Applied),
                 "inside + unblocked validates");
         }
+
+        /// <summary>
+        /// The overlay's repaint reads MASKS, not per-cell queries (main-thread
+        /// hitch fix): the masks must equal the per-cell reads on EVERY origin
+        /// of the grid — the picture may never drift from the rule. The fixture
+        /// deliberately mixes the four register cases: a completed own anchor,
+        /// a second completed own building, an ACTIVE own site (blocks spacing,
+        /// never anchors — D-108) and an enemy building (blocks spacing for
+        /// everyone, anchors for nobody here).
+        /// </summary>
+        [Test]
+        public void FillBuildZoneMasks_MatchesThePerCellReads_OnEveryCell()
+        {
+            var entities = new EntityManager(64);
+            var economy = new EconomySystem(entities, 5000);
+            var costField = new CostField(ConstructionSystem.GridSize, ConstructionSystem.GridSize);
+            var construction = new ConstructionSystem(entities, economy, costField);
+            var kernel = new SimulationKernel(new SimRandom(42UL));
+            kernel.RegisterSystem(economy);
+            kernel.RegisterSystem(construction);
+            kernel.Start();
+
+            Assert.That(construction.PlaceCompletedBuilding(Slot, DefHQAlliance, 4, 4).IsValid, Is.True, "own HQ anchor");
+            Assert.That(construction.PlaceCompletedBuilding(Slot, DefPowerAlliance, 13, 13).IsValid, Is.True, "own completed Power anchor");
+            Assert.That(construction.TryPlaceBuilding(Slot, DefPowerAlliance, 20, 4), Is.True, "active own site");
+            Assert.That(construction.PlaceCompletedBuilding(1, 20, 40, 40).IsValid, Is.True, "enemy HQ (Legion def 20)");
+
+            int size = ConstructionSystem.GridSize;
+            var influence = new bool[size * size];
+            var spacingBlocked = new bool[size * size];
+            construction.FillBuildZoneMasks(Slot, influence, spacingBlocked);
+
+            int mismatches = 0;
+            for (int y = 0; y < size; y++)
+            {
+                for (int x = 0; x < size; x++)
+                {
+                    int cell = y * size + x;
+                    bool expectedInfluence = construction.IsInsideBuildInfluence(Slot, x, y);
+                    bool expectedBlocked = !construction.HasMinimumBuildingSpacing(x, y);
+                    if (influence[cell] != expectedInfluence || spacingBlocked[cell] != expectedBlocked)
+                    {
+                        if (mismatches++ < 5)
+                        {
+                            TestContext.Out.WriteLine(
+                                $"({x},{y}): mask=({influence[cell]},{spacingBlocked[cell]}) direct=({expectedInfluence},{expectedBlocked})");
+                        }
+                    }
+                }
+            }
+            Assert.That(mismatches, Is.EqualTo(0), "the repaint masks must equal the per-cell reads on every origin of the grid");
+        }
     }
 }
diff --git a/Assets/_Project/Scripts/Presentation/UI/BuildZoneOverlayView.cs b/Assets/_Project/Scripts/Presentation/UI/BuildZoneOverlayView.cs
index cb9d998..dab46e7 100644
--- a/Assets/_Project/Scripts/Presentation/UI/BuildZoneOverlayView.cs
+++ b/Assets/_Project/Scripts/Presentation/UI/BuildZoneOverlayView.cs
@@ -84,6 +84,12 @@ public sealed class BuildZoneOverlayView : MonoBehaviour
         private Material _material;
         private GameObject _quad;
         private float _nextRepaintTime;
+        // Repaint scratch (21.4 follow-up): the two zone masks are filled by
+        // the construction system in ONE register pass per repaint — the
+        // per-texel query loop used to rescan that register per texel, which
+        // hitched the main thread four times a second while placing.
+        private bool[] _influenceMask;
+        private bool[] _spacingBlockedMask;
 
         private void Awake()
         {
@@ -121,24 +127,29 @@ private void LateUpdate()
         /// whose 3x3 footprint would leave the map stay clear as well —
         /// painting them buildable would promise a placement the validator
         /// rejects on bounds alone (map geometry, not the zone rule).
+        /// The two masks come from the construction system pre-filled for the
+        /// whole grid (<see cref="ConstructionSystem.FillBuildZoneMasks"/>) —
+        /// texels only read them.
         /// </summary>
         private void Repaint(ConstructionSystem construction, byte viewerSlot)
         {
             int size = ConstructionSystem.GridSize;
             int lastOrigin = size - SimDefinitions.BuildingFootprintCells;
 
+            EnsureMasks(size);
+            construction.FillBuildZoneMasks(viewerSlot, _influenceMask, _spacingBlockedMask);
+
             for (int y = 0; y < size; y++)
             {
                 int row = y * size;
                 for (int x = 0; x < size; x++)
                 {
                     Color32 color = Clear;
-                    if (x <= lastOrigin && y <= lastOrigin
-                        && construction.IsInsideBuildInfluence(viewerSlot, x, y))
+                    if (x <= lastOrigin && y <= lastOrigin && _influenceMask[row + x])
                     {
-                        color = construction.HasMinimumBuildingSpacing(x, y)
-                            ? _buildableColor
-                            : _spacingBlockedColor;
+                        color = _spacingBlockedMask[row + x]
+                            ? _spacingBlockedColor
+                            : _buildableColor;
                     }
                     _pixels[row + x] = color;
                 }
@@ -148,6 +159,15 @@ private void Repaint(ConstructionSystem construction, byte viewerSlot)
             _texture.Apply();
         }
 
+        private void EnsureMasks(int size)
+        {
+            if (_influenceMask == null || _influenceMask.Length != size * size)
+            {
+                _influenceMask = new bool[size * size];
+                _spacingBlockedMask = new bool[size * size];
+            }
+        }
+
         /// <summary>
         /// Builds texture, material and quad lazily on the first show.
         /// Everything is runtime-generated and HideAndDontSave — the slice
diff --git a/Assets/_Project/Scripts/Simulation/Construction/ConstructionSystem.cs b/Assets/_Project/Scripts/Simulation/Construction/ConstructionSystem.cs
index 53e0510..5c976ab 100644
--- a/Assets/_Project/Scripts/Simulation/Construction/ConstructionSystem.cs
+++ b/Assets/_Project/Scripts/Simulation/Construction/ConstructionSystem.cs
@@ -1177,6 +1177,96 @@ public bool HasMinimumBuildingSpacing(int originX, int originY)
             return true;
         }
 
+        /// <summary>
+        /// The build-zone overlay's two reads, hoisted from PER TEXEL to ONE
+        /// PASS (21.4 follow-up — the per-cell calls of
+        /// <see cref="IsInsideBuildInfluence"/> and
+        /// <see cref="HasMinimumBuildingSpacing"/> cost the overlay several
+        /// hundred million operations per repaint: every texel rescanned the
+        /// placement register, and each entry paid a linear site-register
+        /// probe for the finished-only rule. At the 4 Hz repaint cadence
+        /// while a placement ghost is armed, that was a main-thread hitch
+        /// four times a second in exactly the phase the player was placing).
+        /// <para>
+        /// The fill is EXACT, not an approximation: both predicates are
+        /// rectangle-Chebyshev distances between uniform f×f footprints, so
+        /// the set of candidate origins they answer for is a square around
+        /// each register entry. With RectDist(o, p) = max(0, |o−p| − (f−1))
+        /// per axis: inside-influence ⟺ |o−a| ≤ BuildInfluenceRadiusCells +
+        /// f − 1 per axis; spacing-blocked ⟺ |o−b| ≤
+        /// MinimumBuildingDistanceCells + f − 2 per axis. The per-cell reads
+        /// stay the validators' own path — this mask exists for the overlay's
+        /// repaint loop only, and the edit-mode suite pins their equivalence
+        /// over the full grid (BuildZoneOverlayQueryTests).
+        /// </para>
+        /// <para>
+        /// Filters mirror the per-cell reads precisely: influence anchors are
+        /// own, living, COMPLETED placements (a site never anchors — D-108);
+        /// spacing blockers are every active site and every placement with a
+        /// living entity, of ANY owner. Both masks are cleared first and must
+        /// cover the full grid (GridSize × GridSize).
+        /// </para>
+        /// </summary>
+        public void FillBuildZoneMasks(byte playerSlot, bool[] influenceMask, bool[] spacingBlockedMask)
+        {
+            int size = GridSize;
+            if (influenceMask == null || influenceMask.Length < size * size)
+            {
+                throw new ArgumentException("influenceMask must cover the full grid", nameof(influenceMask));
+            }
+            if (spacingBlockedMask == null || spacingBlockedMask.Length < size * size)
+            {
+                throw new ArgumentException("spacingBlockedMask must cover the full grid", nameof(spacingBlockedMask));
+            }
+            Array.Clear(influenceMask, 0, size * size);
+            Array.Clear(spacingBlockedMask, 0, size * size);
+
+            int f = SimDefinitions.BuildingFootprintCells;
+            int influenceHalf = BuildInfluenceRadiusCells + f - 1;
+            int spacingHalf = MinimumBuildingDistanceCells + f - 2;
+
+            for (int i = 0; i < MaxSites; i++)
+            {
+                ref readonly SiteState site = ref _sites[i];
+                if (site.IsActive)
+                {
+                    FillOriginBox(spacingBlockedMask, size, site.OriginX, site.OriginY, spacingHalf);
+                }
+            }
+
+            for (int i = 0; i < MaxBuildings; i++)
+            {
+                ref readonly PlacementState placement = ref _buildings[i];
+                if (!placement.IsActive) continue;
+                EntityId id = UnitCommandStateView.ToEntityId(placement.RawEntityId);
+                if (_entityManager.IsValid(id))
+                {
+                    FillOriginBox(spacingBlockedMask, size, placement.OriginX, placement.OriginY, spacingHalf);
+                }
+
+                if (IsActiveSite(id)) continue; // finished only: a site never extends the zone
+                if (!_entityManager.TryGetUnit(id, out UnitState unit) || unit.PlayerId != playerSlot) continue;
+                FillOriginBox(influenceMask, size, placement.OriginX, placement.OriginY, influenceHalf);
+            }
+        }
+
+        /// <summary>Marks the clamped square of candidate origins (cx±half, cy±half) in a row-major grid mask.</summary>
+        private static void FillOriginBox(bool[] mask, int size, int centreX, int centreY, int half)
+        {
+            int minX = Math.Max(0, centreX - half);
+            int maxX = Math.Min(size - 1, centreX + half);
+            int minY = Math.Max(0, centreY - half);
+            int maxY = Math.Min(size - 1, centreY + half);
+            for (int y = minY; y <= maxY; y++)
+            {
+                int row = y * size;
+                for (int x = minX; x <= maxX; x++)
+                {
+                    mask[row + x] = true;
+                }
+            }
+        }
+
         private bool HasValidFieldSpacing(UnitRole role, int originX, int originY)
         {
             bool refineryHasFieldInRange = false;
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7c5f02c..6044afe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -269,6 +269,17 @@ die Versionierung folgt (in der aktuellen Doku-Phase) dem Dokumentationsstand de
   bei 2 AE/Tick, bis eine gespielte Balance-Kalibrierung belastbare Werte gibt
 
 ### Behoben
+- **Ruckler in der Bauphase: das Baubereich-Overlay hat den Hauptthread gebremst.**
+  Das 21.4-Overlay fragte die beiden Zonen-Reads (`IsInsideBuildInfluence`,
+  `HasMinimumBuildingSpacing`) **pro Texel** ab — jeder Texel scannte das
+  Platzierungsregister erneut, jeder Eintrag zahlte eine lineare
+  Baustellen-Probe. Das sind mehrere hundert Millionen Operationen pro
+  Repaint, viermal pro Sekunde, ausgerechnet während ein Bau-Ghost armiert
+  ist — Spielbericht: „komisch verzögert". Jetzt füllt
+  `ConstructionSystem.FillBuildZoneMasks` beide Masken in **einem** Pass
+  (exakte Rechtecks-Chebyshev-Boxen statt Texel-Scan, Äquivalenz über das
+  ganze Raster gepinnt); der Repaint liest nur noch. Selbes Bild, kein
+  Hitch. Simulationsverhalten unverändert, keine Baseline bewegt
 - **#85: Die KI erntet nicht länger endlos auf dem leeren Feld.** Aus dem
   Betatest vom 10.08.2026: die KI kam nach Erschöpfung ihres Startvorkommens
   wirtschaftlich zum Stillstand. Das war kein Strategiemangel, sondern ein