diff --git a/project.godot b/project.godot
index dfd096a..0ef2aea 100644
--- a/project.godot
+++ b/project.godot
@@ -110,3 +110,7 @@ interact={
 3d_render/layer_2="HUD"
 3d_physics/layer_1="Physics"
 3d_physics/layer_2="Interactive"
+
+[rendering]
+
+driver/threads/thread_model=2
diff --git a/src/props/wall_switch/wall_switch.gd b/src/props/wall_switch/wall_switch.gd
index c05f6ae..43fc102 100644
--- a/src/props/wall_switch/wall_switch.gd
+++ b/src/props/wall_switch/wall_switch.gd
@@ -27,13 +27,6 @@ func enable() -> void:
 	interactive.enabled = true
 
 
-func _on_gunk_body_painted() -> void:
-	if not enabled:
-		var clear_total := gunk_body.get_clear_total()
-		if clear_total >= CLEAR_THRESHOLD:
-			enable()
-
-
 func _activate() -> void:
 	animation_player.play("activate")
 	activated.emit()
@@ -43,3 +36,8 @@ func _activate() -> void:
 
 func _animation_finished(_anim_name: StringName) -> void:
 	interactive.enabled = true
+
+
+func _on_gunk_body_clear_total_updated(clear_total: float) -> void:
+	if not enabled and clear_total >= CLEAR_THRESHOLD:
+		enable()
diff --git a/src/props/wall_switch/wall_switch.tscn b/src/props/wall_switch/wall_switch.tscn
index 1283bc2..551488f 100644
--- a/src/props/wall_switch/wall_switch.tscn
+++ b/src/props/wall_switch/wall_switch.tscn
@@ -241,5 +241,5 @@ script = ExtResource("10_qw6jt")
 shape = SubResource("BoxShape3D_6maql")
 
 [connection signal="animation_finished" from="AnimationPlayer" to="." method="_animation_finished"]
-[connection signal="painted" from="GunkBody" to="." method="_on_gunk_body_painted"]
+[connection signal="clear_total_updated" from="GunkBody" to="." method="_on_gunk_body_clear_total_updated"]
 [connection signal="activated" from="Interactive" to="." method="_activate"]
diff --git a/src/world/gunk_body/gunk_body.gd b/src/world/gunk_body/gunk_body.gd
index efda01a..2f95c52 100644
--- a/src/world/gunk_body/gunk_body.gd
+++ b/src/world/gunk_body/gunk_body.gd
@@ -1,12 +1,14 @@
 class_name GunkBody extends StaticBody3D
 ## StaticBody3D with an associated "gunkable" mesh.
 
-signal painted
+## Emitted from the main thread after the clear total is asynchronously updated.
+signal clear_total_updated(clear_total: float)
 
 const CONTINUITY_LIMIT := 16
 const BUFFER_LIMIT := 3
 const FACE_EPSILON := 0.01
 const MASK_COLOR := Color.RED
+const CLEAR_TOTAL_EPSILON := 1
 
 @export var mask_dim := 256
 @export var mesh_instance: MeshInstance3D
@@ -20,7 +22,14 @@ var _multiline_buffer := PackedVector2Array()
 var _multiline_width := 1.0
 
 var _clear_total := 0.0
-var _clear_total_dirty := true
+var _prev_clear_total := 0.0
+
+# _clear_total is async computed in separate thread
+var _mask_tx: Texture2D
+var _thread: Thread
+var _mutex: Mutex = Mutex.new()
+var _semaphore: Semaphore = Semaphore.new()
+var _exit_thread := false
 
 @onready var mesh: ArrayMesh = mesh_instance.mesh
 @onready var gunk_mat: ShaderMaterial = mesh_instance.mesh.surface_get_material(0).next_pass
@@ -36,6 +45,56 @@ func _ready() -> void:
 	gunk_mat.set_shader_parameter("gunk_mask", mask_viewport.get_texture())
 	meshtool.create_from_surface(mesh, 0)
 
+	_thread = Thread.new()
+	_thread.start(_async_compute_clear_total)
+
+
+func _trigger_recompute_deferred() -> void:
+	_mutex.lock()
+	_mask_tx = mask_viewport.get_texture()
+	#_mask_img = mask_viewport.get_texture().get_image()
+	_mutex.unlock()
+	_semaphore.post()
+
+
+func _async_compute_clear_total() -> void:
+	while true:
+		_semaphore.wait()
+
+		# check exit flag
+		_mutex.lock()
+		var exiting := _exit_thread
+		_mutex.unlock()
+
+		if exiting:
+			break
+
+		# Get mask from GPU
+		# TODO: does this need mutex protection?
+		_mutex.lock()
+		var mask_img := _mask_tx.get_image()
+		_mutex.unlock()
+		mask_img.convert(Image.FORMAT_R8)
+		var px_data := mask_img.get_data()
+		var px_sum := 0
+		for px in px_data:
+			px_sum += px
+		var new_total := px_sum / 255.0
+
+		# Write total
+		_mutex.lock()
+		_clear_total = new_total
+		_mutex.unlock()
+
+
+func _exit_tree() -> void:
+	# Flag processing flag to cleanly exit
+	_mutex.lock()
+	_exit_thread = true
+	_mutex.unlock()
+	_semaphore.post()
+	_thread.wait_to_finish()
+
 
 func clear_all() -> void:
 	mask_control.queue_draw(
@@ -51,16 +110,12 @@ func clear_all() -> void:
 ## This will use a cached result unless the mask has been painted since the last calculation.
 ## Be aware that cache misses are potentially quite expensive.
 func get_clear_total() -> float:
-	if _clear_total_dirty:
-		var mask_img := mask_viewport.get_texture().get_image()
-		mask_img.convert(Image.FORMAT_R8)
-		var px_data := mask_img.get_data()
-		var px_sum := 0
-		for px in px_data:
-			px_sum += px
-		_clear_total = px_sum / 255.0
-		_clear_total_dirty = false
-	return _clear_total
+	# Protect with mutex
+	_mutex.lock()
+	var total := _clear_total
+	_mutex.unlock()
+
+	return total
 
 
 ## Transform cartesian coordinates to barycentric wrt the given triangle.
@@ -182,6 +237,14 @@ func add_to_multiline(
 
 
 func _process(_delta: float) -> void:
+	# Check if processing thread has updated the clear total
+	_mutex.lock()
+	var new_total := _clear_total
+	_mutex.unlock()
+	if abs(new_total - _prev_clear_total) > CLEAR_TOTAL_EPSILON:
+		clear_total_updated.emit(new_total)
+		_prev_clear_total = new_total
+
 	# If paint_continuous wasn't called last frame, stop the current polyline.
 	if not _continued_paint_this_frame:
 		_polyline_buffer = []
@@ -199,5 +262,5 @@ func _process(_delta: float) -> void:
 
 
 func _on_mask_painted() -> void:
-	_clear_total_dirty = true
-	painted.emit()
+	# XXX any problem with posting each frame?
+	call_deferred("_trigger_recompute_deferred")