diff --git a/guide/age-of-dwarves/src/simulation/simulation.worker.ts b/guide/age-of-dwarves/src/simulation/simulation.worker.ts
index fe899ae2..155dcfab 100644
--- a/guide/age-of-dwarves/src/simulation/simulation.worker.ts
+++ b/guide/age-of-dwarves/src/simulation/simulation.worker.ts
@@ -151,16 +151,19 @@ function stepOneTurn(state: ScenarioState, scenarioTurn: number, specJson: strin
     state.wasmGrid = jsGridToWasm(state.grid)
   }
   wasmPhysicsStep(state.physics, state.ecology, state.wasmGrid, absTurn, state.worldSeed, specJson)
-  // Sync WASM grid back to JS for stats computation
-  state.grid = wasmGridToJs(state.wasmGrid)
+  // Compute stats in Rust — avoids full 960-tile JSON deserialization every turn.
   const prevStats = state.stats.length > 0 ? state.stats[state.stats.length - 1] : undefined
-  const stats = computeTurnStats(state.grid, prevStats)
+  const stats: TurnStats = JSON.parse(
+    state.wasmGrid.computeStatsJson(prevStats?.avg_temp ?? 0.5, prevStats?.avg_moisture ?? 0.5)
+  )
   state.cursorScenarioTurn = scenarioTurn + 1
-  // WASM physics doesn't return events; events are tracked in stats
   return { stats, events: [] }
 }
 
 function addCheckpoint(state: ScenarioState, turn: number): void {
+  // Sync JS grid from WASM before checkpointing — checkpoints are infrequent (every 100 turns)
+  // so the toJSON cost here is acceptable. The per-turn hot path avoids this.
+  state.grid = wasmGridToJs(state.wasmGrid)
   state.checkpoints.set(turn, cloneGridState(state.grid))
 
   if (state.checkpoints.size > MAX_CHECKPOINTS) {