diff --git a/.jules/bolt.md b/.jules/bolt.md
index 58ed3de2..191736f9 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -73,3 +73,7 @@
 ## 2026-04-17 - ORM Counting vs func.count().scalar()
 **Learning:** Using `db.query(Model).filter(...).count()` can be slower and have more ORM overhead than `db.query(func.count(Model.id)).filter(...).scalar() or 0` or doing an early `.first()` exit.
 **Action:** When counting records or verifying existence, prefer early `.first()` exits combined with `func.count().scalar()` for performance in high-traffic APIs.
+
+## 2026-04-20 - Async File I/O in Voice Submission
+**Learning:** Saving audio recordings (up to 10MB) synchronously in a FastAPI async endpoint blocks the main event loop, significantly increasing tail latency for all concurrent users during high-traffic periods.
+**Action:** Wrap blocking synchronous File I/O operations like `f.write()` in `run_in_threadpool` to offload them to a separate thread, keeping the event loop responsive for other requests.
diff --git a/backend/routers/voice.py b/backend/routers/voice.py
index 5ec6e385..a908f614 100644
--- a/backend/routers/voice.py
+++ b/backend/routers/voice.py
@@ -11,10 +11,12 @@
 import logging
 import os
 import uuid
+import hashlib
 from datetime import datetime, timezone
 
 from backend.database import get_db
 from backend.models import Issue
+from backend.cache import blockchain_last_hash_cache
 from backend.schemas import (
     VoiceTranscriptionResponse,
     TextTranslationRequest,
@@ -248,12 +250,44 @@ async def submit_voice_issue(
         audio_filename = f"{datetime.now(timezone.utc).strftime('%Y%m%d_%H%M%S')}_{uuid.uuid4().hex}{file_extension}"
         audio_file_path = os.path.join(AUDIO_STORAGE_DIR, audio_filename)
         
-        with open(audio_file_path, 'wb') as f:
-            f.write(audio_content)
-        
-        # Store relative path for portability
-        relative_audio_path = os.path.join("data", "audio_recordings", audio_filename)
+        # Performance optimization: Wrap blocking synchronous File I/O in threadpool
+        def _save_audio_file():
+            with open(audio_file_path, 'wb') as f:
+                f.write(audio_content)
+
+        def _delete_audio_file_best_effort():
+            try:
+                if os.path.exists(audio_file_path):
+                    os.remove(audio_file_path)
+            except Exception as cleanup_error:
+                logger.warning("Failed to delete orphaned audio file %s: %s", audio_file_path, cleanup_error)
+
+        await run_in_threadpool(_save_audio_file)
         
+        try:
+            # Store relative path for portability
+            relative_audio_path = os.path.join("data", "audio_recordings", audio_filename)
+            
+            # Blockchain feature: calculate integrity hash for the report
+            # Performance Boost: Use thread-safe cache to eliminate DB query for last hash
+            prev_hash = blockchain_last_hash_cache.get("last_hash")
+            if prev_hash is None:
+                # Cache miss: Fetch only the last hash from DB
+                prev_issue = await run_in_threadpool(
+                    lambda: db.query(Issue.integrity_hash).order_by(Issue.id.desc()).first()
+                )
+                prev_hash = prev_issue[0] if prev_issue and prev_issue[0] else ""
+                blockchain_last_hash_cache.set(data=prev_hash, key="last_hash")
+
+            # Simple but effective SHA-256 chaining
+            # Format must match backend/routers/issues.py for a consistent chain
+            hash_content = f"{final_description}|{issue_category.value}|{prev_hash}"
+        except Exception:
+            db.rollback()
+            await run_in_threadpool(_delete_audio_file_best_effort)
+            raise
+        integrity_hash = hashlib.sha256(hash_content.encode()).hexdigest()
+
         # Create issue in database
         reference_id = generate_reference_id()
         
@@ -267,6 +301,8 @@ async def submit_voice_issue(
             location=location,
             source='voice',
             status='open',
+            integrity_hash=integrity_hash,
+            previous_integrity_hash=prev_hash,
             # Voice-specific fields
             submission_type='voice',
             original_language=voice_result.get('source_language'),
@@ -280,6 +316,9 @@ async def submit_voice_issue(
         db.commit()
         db.refresh(new_issue)
         
+        # Update cache for next report AFTER successful DB commit
+        blockchain_last_hash_cache.set(data=integrity_hash, key="last_hash")
+
         logger.info(f"Voice issue created: ID={new_issue.id}, Language={voice_result.get('source_language')}, Confidence={voice_result.get('confidence')}")
         
         return IssueCreateResponse(