diff --git a/src/viewer/SceneManager.ts b/src/viewer/SceneManager.ts
index 8b7e0fb7..057fa8c9 100644
--- a/src/viewer/SceneManager.ts
+++ b/src/viewer/SceneManager.ts
@@ -68,6 +68,7 @@ import { clippedRaycast } from './clipRaycast'
 import { clipTree } from './clipTree'
 import { CSS2DObject } from 'three/addons/renderers/CSS2DRenderer.js'
 import { VolumeLayer } from './volumeLayer'
+import { clusterIndices, MotionLatch } from './lod'
 import type { VolumeDeviation } from '../core/deviation/volume'
 
 declare module 'three' {
@@ -404,6 +405,11 @@ export class SceneManager {
    *  A ghost until the workspace says otherwise, which is how it starts. */
   private nominalGhost = true
 
+  /** The scan's coarse stand-in while the camera moves — see lod.ts. */
+  private lodLatch = new MotionLatch()
+  private lodCache: { geometry: THREE.BufferGeometry; position: ArrayLike<number>; index: ArrayLike<number>; proxy: THREE.Mesh | null } | null = null
+  private lodShown: { proxy: THREE.Mesh; geometry: THREE.BufferGeometry } | null = null
+
   /** Scratch for picking, which runs every frame the cursor moves: the
    *  barycentric corners and difference vectors, and (in D) the hit point
    *  carried into the part's frame. */
@@ -467,6 +473,7 @@ export class SceneManager {
         this.grips.putAwayWhile(this.marking.gestureArmed())
         this.overlays.setPreviewBrushing(this.marking.gestureArmed())
         this.updateHover()
+        this.stepLod()
         // Anything held at a fixed size on screen — the picked-point markers —
         // has to be re-scaled when the zoom changes. A no-op on the frames it
         // has not.
@@ -2944,7 +2951,73 @@ export class SceneManager {
     this.nominalMesh = null
   }
 
+  /** Swap the scan for its coarse proxy while the camera moves, and back
+   *  once it settles — see lod.ts. The full mesh is kept, only not drawn:
+   *  its draw range is emptied, so picks still go through it. */
+  private stepLod(): void {
+    const moving = this.lodLatch.update(this.camera.matrixWorld.elements, performance.now())
+    const want = moving && this.mesh ? this.lodProxyFor(this.mesh) : null
+    if (want === this.lodShown?.proxy) return
+    this.hideLod()
+    if (want && this.mesh) {
+      const geometry = this.mesh.geometry as THREE.BufferGeometry
+      this.mesh.add(want)
+      geometry.setDrawRange(0, 0)
+      this.lodShown = { proxy: want, geometry }
+    }
+    this.invalidate()
+  }
+
+  private hideLod(): void {
+    if (!this.lodShown) return
+    this.lodShown.geometry.setDrawRange(0, Infinity)
+    this.lodShown.proxy.removeFromParent()
+    this.lodShown = null
+  }
+
+  /** The proxy of the scan as it stands, made on first need and kept until
+   *  its arrays change hands; null for a scan small enough to draw whole. */
+  private lodProxyFor(mesh: THREE.Mesh): THREE.Mesh | null {
+    const geometry = mesh.geometry as THREE.BufferGeometry
+    const position = geometry.getAttribute('position')
+    const index = geometry.index
+    if (!index || !position) return null
+    const c = this.lodCache
+    if (c && c.geometry === geometry && c.position === position.array && c.index === index.array) return c.proxy
+    this.dropLod()
+    const cut = clusterIndices(position.array as Float32Array, index.array as Uint32Array)
+    let proxy: THREE.Mesh | null = null
+    if (cut) {
+      const shell = new THREE.BufferGeometry()
+      shell.setIndex(new THREE.BufferAttribute(cut, 1))
+      proxy = new THREE.Mesh(shell, mesh.material)
+      proxy.frustumCulled = false
+      proxy.raycast = () => {}
+      // The scan's attributes are borrowed as drawn, so colours, deviation
+      // and paint are whatever the full mesh carries this frame.
+      const p = proxy
+      p.onBeforeRender = (...args) => {
+        const full = mesh.geometry as THREE.BufferGeometry
+        for (const name of Object.keys(shell.attributes)) if (!full.getAttribute(name)) shell.deleteAttribute(name)
+        for (const [name, attr] of Object.entries(full.attributes)) {
+          if (shell.getAttribute(name) !== attr) shell.setAttribute(name, attr)
+        }
+        p.material = mesh.material
+        mesh.onBeforeRender(...args)
+      }
+    }
+    this.lodCache = { geometry, position: position.array, index: index.array, proxy }
+    return proxy
+  }
+
+  private dropLod(): void {
+    this.hideLod()
+    this.lodCache?.proxy?.geometry.dispose()
+    this.lodCache = null
+  }
+
   private disposeMesh(): void {
+    this.dropLod()
     this.setEditedMesh(null)
     this.regions.detach()
     this.marking.meshDisposed()
diff --git a/src/viewer/lod.ts b/src/viewer/lod.ts
new file mode 100644
index 00000000..e7c4d3a3
--- /dev/null
+++ b/src/viewer/lod.ts
@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// Level of detail for the scan while the camera moves. A scan of a few million
+// triangles orbits at a crawl, so while the view is in motion a coarse proxy
+// is drawn in its place and the full mesh comes back once the view settles.
+// The proxy is only an index buffer over the scan's own vertices — colours,
+// deviation, paint and texture ride along unchanged — and it is never
+// raycast: picking and measuring always see full detail.
+
+/** Scans above this many triangles get a proxy, cut to about this many. */
+export const LOD_TRIANGLES = 400_000
+
+/** How long the view has to hold still before full detail comes back. */
+export const LOD_SETTLE_MS = 180
+
+/**
+ * Triangles of a coarser mesh by vertex clustering: the bounding box is cut
+ * into a grid, every vertex stands in for the first one met in its cell, and
+ * triangles that collapse are dropped. The grid is resized until the result
+ * is near `target` triangles. Null when the mesh is small enough already.
+ */
+export function clusterIndices(positions: Float32Array, indices: Uint32Array, target = LOD_TRIANGLES): Uint32Array | null {
+  const tris = indices.length / 3
+  if (tris <= target) return null
+  const n = positions.length / 3
+  let minX = Infinity, minY = Infinity, minZ = Infinity
+  let maxX = -Infinity, maxY = -Infinity, maxZ = -Infinity
+  for (let i = 0; i < n; i++) {
+    const x = positions[i * 3], y = positions[i * 3 + 1], z = positions[i * 3 + 2]
+    if (x < minX) minX = x
+    if (x > maxX) maxX = x
+    if (y < minY) minY = y
+    if (y > maxY) maxY = y
+    if (z < minZ) minZ = z
+    if (z > maxZ) maxZ = z
+  }
+  const extent = Math.max(maxX - minX, maxY - minY, maxZ - minZ, 1e-9)
+  const rep = new Int32Array(n)
+  const cells = new Map<number, number>()
+  // A surface meets about as many cells as the grid's side squared, and a
+  // mesh has about twice as many triangles as vertices.
+  let side = Math.max(2, Math.sqrt(target / 2))
+  let out: Uint32Array = new Uint32Array(0)
+  for (let pass = 0; pass < 3; pass++) {
+    const k = Math.min(Math.floor(side), 2 ** 20 - 1)
+    const s = k / extent
+    cells.clear()
+    for (let i = 0; i < n; i++) {
+      const cx = Math.min(k, Math.floor((positions[i * 3] - minX) * s))
+      const cy = Math.min(k, Math.floor((positions[i * 3 + 1] - minY) * s))
+      const cz = Math.min(k, Math.floor((positions[i * 3 + 2] - minZ) * s))
+      const key = (cx * (k + 1) + cy) * (k + 1) + cz
+      let r = cells.get(key)
+      if (r === undefined) cells.set(key, (r = i))
+      rep[i] = r
+    }
+    const kept: number[] = []
+    for (let t = 0; t < indices.length; t += 3) {
+      const a = rep[indices[t]], b = rep[indices[t + 1]], c = rep[indices[t + 2]]
+      if (a !== b && b !== c && a !== c) kept.push(a, b, c)
+    }
+    out = Uint32Array.from(kept)
+    const got = out.length / 3
+    if (got > target * 0.7 && got <= target * 1.1) break
+    side *= Math.sqrt(target / Math.max(got, 1))
+  }
+  return out.length / 3 < tris ? out : null
+}
+
+/** Whether the camera is on the move: true from the frame its matrix changes
+ *  until it has held still for `settleMs`. */
+export class MotionLatch {
+  private last: number[] | null = null
+  private movedAt = -Infinity
+
+  constructor(private settleMs = LOD_SETTLE_MS) {}
+
+  update(matrix: ArrayLike<number>, now: number): boolean {
+    const m = Array.from(matrix)
+    if (this.last && m.some((v, i) => v !== this.last![i])) this.movedAt = now
+    this.last = m
+    return now - this.movedAt < this.settleMs
+  }
+}
diff --git a/tests/lod.test.ts b/tests/lod.test.ts
new file mode 100644
index 00000000..ae613e61
--- /dev/null
+++ b/tests/lod.test.ts
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+import { describe, expect, it } from 'vitest'
+import { clusterIndices, MotionLatch } from '../src/viewer/lod'
+
+/** A flat grid of side `n` quads, two triangles each. */
+function grid(n: number): { positions: Float32Array; indices: Uint32Array } {
+  const positions = new Float32Array((n + 1) * (n + 1) * 3)
+  for (let y = 0; y <= n; y++) for (let x = 0; x <= n; x++) positions.set([x, y, 0], (y * (n + 1) + x) * 3)
+  const indices: number[] = []
+  for (let y = 0; y < n; y++) for (let x = 0; x < n; x++) {
+    const a = y * (n + 1) + x
+    indices.push(a, a + 1, a + n + 1, a + 1, a + n + 2, a + n + 1)
+  }
+  return { positions, indices: Uint32Array.from(indices) }
+}
+
+describe('level of detail', () => {
+  it('leaves a small mesh alone', () => {
+    const g = grid(10)
+    expect(clusterIndices(g.positions, g.indices, 1000)).toBeNull()
+  })
+
+  it('cuts a big mesh to near the budget over its own vertices', () => {
+    const g = grid(200)
+    const cut = clusterIndices(g.positions, g.indices, 5000)!
+    const tris = cut.length / 3
+    expect(tris).toBeLessThan(g.indices.length / 3)
+    expect(tris).toBeGreaterThan(2000)
+    expect(tris).toBeLessThan(8000)
+    const vertices = g.positions.length / 3
+    for (let t = 0; t < cut.length; t += 3) {
+      expect(cut[t] < vertices && cut[t] !== cut[t + 1] && cut[t + 1] !== cut[t + 2]).toBe(true)
+    }
+  })
+
+  it('holds motion until the camera settles', () => {
+    const latch = new MotionLatch(100)
+    expect(latch.update([1, 0], 0)).toBe(false)
+    expect(latch.update([2, 0], 10)).toBe(true)
+    expect(latch.update([2, 0], 90)).toBe(true)
+    expect(latch.update([2, 0], 111)).toBe(false)
+  })
+})