diff --git a/src/collision.h b/src/collision.h
index e6f690fe..7a87da79 100644
--- a/src/collision.h
+++ b/src/collision.h
@@ -18,11 +18,6 @@ static inline bool Collision_matchesTarget(DataWin* dataWin, Instance* inst, int
     return VM_isObjectOrDescendant(dataWin, inst->objectIndex, target);
 }
 
-typedef struct {
-    GMLReal left, right, top, bottom;
-    bool valid;
-} InstanceBBox;
-
 // Returns the collision sprite for an instance (mask sprite if set, else display sprite)
 static inline Sprite* Collision_getSprite(DataWin* dataWin, Instance* inst) {
     int32_t sprIdx = (inst->maskIndex >= 0) ? inst->maskIndex : inst->spriteIndex;
@@ -32,8 +27,15 @@ static inline Sprite* Collision_getSprite(DataWin* dataWin, Instance* inst) {
 
 // Computes the axis-aligned bounding box for an instance using its collision sprite
 static inline InstanceBBox Collision_computeBBox(Runner* runner, Instance* inst) {
+    // Fast path: return cached AABB when nothing bbox-affecting has changed.
+    if (inst->bboxCacheValid) return inst->cachedBBox;
+
     Sprite* spr = Collision_getSprite(runner->dataWin, inst);
-    if (spr == nullptr) return (InstanceBBox){0, 0, 0, 0, false};
+    if (spr == nullptr) {
+        inst->cachedBBox      = (InstanceBBox){0, 0, 0, 0, false};
+        inst->bboxCacheValid  = true;
+        return inst->cachedBBox;
+    }
 
     GMLReal marginL = (GMLReal) spr->marginLeft;
     GMLReal marginR = (GMLReal) (spr->marginRight + 1);
@@ -42,6 +44,7 @@ static inline InstanceBBox Collision_computeBBox(Runner* runner, Instance* inst)
     GMLReal originX = (GMLReal) spr->originX;
     GMLReal originY = (GMLReal) spr->originY;
 
+    InstanceBBox result;
     GMLReal left, right, top, bottom;
     if (GMLReal_fabs(inst->imageAngle) > 0.0001) {
         // Compute rotated AABB: transform the 4 corners of the unrotated bbox
@@ -62,13 +65,10 @@ static inline InstanceBBox Collision_computeBBox(Runner* runner, Instance* inst)
         cx[2] = cs * lx0 + sn * ly1;  cy[2] = -sn * lx0 + cs * ly1;
         cx[3] = cs * lx1 + sn * ly1;  cy[3] = -sn * lx1 + cs * ly1;
 
-        GMLReal minX = cx[0], maxX = cx[0], minY = cy[0], maxY = cy[0];
-        for (int c = 1; 4 > c; c++) {
-            if (minX > cx[c]) minX = cx[c];
-            if (cx[c] > maxX) maxX = cx[c];
-            if (minY > cy[c]) minY = cy[c];
-            if (cy[c] > maxY) maxY = cy[c];
-        }
+        GMLReal minX = GMLReal_fmin(GMLReal_fmin(cx[0], cx[1]), GMLReal_fmin(cx[2], cx[3]));
+        GMLReal maxX = GMLReal_fmax(GMLReal_fmax(cx[0], cx[1]), GMLReal_fmax(cx[2], cx[3]));
+        GMLReal minY = GMLReal_fmin(GMLReal_fmin(cy[0], cy[1]), GMLReal_fmin(cy[2], cy[3]));
+        GMLReal maxY = GMLReal_fmax(GMLReal_fmax(cy[0], cy[1]), GMLReal_fmax(cy[2], cy[3]));
 
         left   = inst->x + minX;
         right  = inst->x + maxX;
@@ -82,8 +82,14 @@ static inline InstanceBBox Collision_computeBBox(Runner* runner, Instance* inst)
         bottom = inst->y + inst->imageYscale * (marginB - originY);
 
         // Normalize if negative scale
-        if (left > right) { GMLReal tmp = left; left = right; right = tmp; }
-        if (top > bottom) { GMLReal tmp = top; top = bottom; bottom = tmp; }
+        GMLReal tmp_left   = GMLReal_fmin(left, right);
+        GMLReal tmp_right  = GMLReal_fmax(left, right);
+        GMLReal tmp_top    = GMLReal_fmin(top, bottom);
+        GMLReal tmp_bottom = GMLReal_fmax(top, bottom);
+        left   = tmp_left;
+        right  = tmp_right;
+        top    = tmp_top;
+        bottom = tmp_bottom;
     }
 
     if (runner->collisionCompatibilityMode) {
@@ -93,7 +99,11 @@ static inline InstanceBBox Collision_computeBBox(Runner* runner, Instance* inst)
         bottom = GMLReal_bankersRound(bottom);
     }
 
-    return (InstanceBBox){left, right, top, bottom, true};
+    result = (InstanceBBox){left, right, top, bottom, true};
+
+    inst->cachedBBox     = result;
+    inst->bboxCacheValid = true;
+    return result;
 }
 
 static inline bool Collision_hasFrameMasks(Sprite* sprite) {
diff --git a/src/collision_types.h b/src/collision_types.h
new file mode 100644
index 00000000..d3de55b3
--- /dev/null
+++ b/src/collision_types.h
@@ -0,0 +1,7 @@
+#pragma once
+#include "real_type.h"
+
+typedef struct {
+    GMLReal left, right, top, bottom;
+    bool valid;
+} InstanceBBox;
diff --git a/src/instance.c b/src/instance.c
index 4af3e4aa..572e3d09 100644
--- a/src/instance.c
+++ b/src/instance.c
@@ -47,6 +47,7 @@ Instance* Instance_create(uint32_t instanceId, int32_t objectIndex, GMLReal x, G
     inst->gravityDirection = 270.0f;
     inst->pathIndex = -1;
     inst->pathScale = 1.0f;
+    inst->bboxCacheValid = false;
     inst->timelineIndex = -1;
     inst->timelinePosition = 0.0f;
     inst->timelineSpeed = 1.0f;
diff --git a/src/instance.h b/src/instance.h
index 5876fce2..6e91ec3a 100644
--- a/src/instance.h
+++ b/src/instance.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "common.h"
+#include "collision_types.h"
 #include <stdint.h>
 #include "rvalue.h"
 #include "gml_array.h"
@@ -62,6 +63,10 @@ struct Instance {
     float pathXStart;             // origin for relative paths
     float pathYStart;
 
+    // AABB cache for fast path reuse
+    InstanceBBox cachedBBox;
+    bool         bboxCacheValid;
+
     int32_t alarm[GML_ALARM_COUNT];
 
     // Timeline following state
diff --git a/src/spatial_grid.c b/src/spatial_grid.c
index 66c86155..dc04b5d5 100644
--- a/src/spatial_grid.c
+++ b/src/spatial_grid.c
@@ -103,6 +103,8 @@ void SpatialGrid_markInstanceAsDirty(SpatialGrid* grid, Instance* dirtyInstance)
         return;
     }
 
+    dirtyInstance->bboxCacheValid = false;
+
     if (dirtyInstance->spatialGridDirty)
         return;