diff --git a/app/main.py b/app/main.py
index 4492c48..ec564de 100644
--- a/app/main.py
+++ b/app/main.py
@@ -11,6 +11,8 @@
 
 import asyncio
 import collections
+import aiofiles
+import aiofiles.os
 import json
 import logging
 import os
@@ -697,21 +699,12 @@ async def _send_final_summary(call_sid: str):
     logger.info(f"Final summary sent for {call_sid[:12]}")
 
 
-def _write_json_file(path: Path, data: dict):
-    path.write_text(
-        json.dumps(data, ensure_ascii=False, indent=2),
-        encoding="utf-8",
-    )
-
-
 async def _save_call_to_file(call_sid: str, summary: str):
     """Persist call data to /data/calls/ as JSON for later review."""
     state = active_calls.get(call_sid)
     if not state:
         return
 
-    CALLS_DIR.mkdir(parents=True, exist_ok=True)
-
     date_prefix = datetime.utcnow().strftime("%Y%m%d_%H%M%S")
     filename = f"{date_prefix}_{call_sid[:8]}.json"
 
@@ -728,7 +721,9 @@ async def _save_call_to_file(call_sid: str, summary: str):
     }
 
     try:
-        await asyncio.to_thread(_write_json_file, CALLS_DIR / filename, call_data)
+        await aiofiles.os.makedirs(CALLS_DIR, exist_ok=True)
+        async with aiofiles.open(CALLS_DIR / filename, mode="w", encoding="utf-8") as f:
+            await f.write(json.dumps(call_data, ensure_ascii=False, indent=2))
         logger.info(f"Call data saved: /data/calls/{filename}")
     except Exception as exc:
         logger.error(f"Failed to save call data: {exc}")
diff --git a/benchmark_io.py b/benchmark_io.py
new file mode 100644
index 0000000..fc0047e
--- /dev/null
+++ b/benchmark_io.py
@@ -0,0 +1,73 @@
+import asyncio
+import time
+import json
+import os
+import shutil
+from pathlib import Path
+
+# Simulate main.py setup
+CALLS_DIR = Path("/tmp/data/calls")
+
+# Original implementation
+def _write_json_file_orig(path: Path, data: dict):
+    path.write_text(
+        json.dumps(data, ensure_ascii=False, indent=2),
+        encoding="utf-8",
+    )
+
+async def _save_call_orig():
+    # Simulate what runs in the async context
+    CALLS_DIR.mkdir(parents=True, exist_ok=True)
+    filename = f"test_call_orig_{time.time()}.json"
+    call_data = {"test": "data", "padding": "x" * 1000}
+
+    # We want to measure only the part that runs IN THE EVENT LOOP
+    # because that's what blocks other async tasks.
+    start = time.perf_counter()
+    CALLS_DIR.mkdir(parents=True, exist_ok=True)
+    # The actual write is already offloaded, so the blocking part is mkdir and any other sync work before to_thread
+    t_start = time.perf_counter()
+    await asyncio.to_thread(_write_json_file_orig, CALLS_DIR / filename, call_data)
+
+    return time.perf_counter() - start
+
+import aiofiles
+import aiofiles.os
+
+# Optimized implementation
+async def _save_call_opt():
+    filename = f"test_call_opt_{time.time()}.json"
+    call_data = {"test": "data", "padding": "x" * 1000}
+
+    start = time.perf_counter()
+    await aiofiles.os.makedirs(CALLS_DIR, exist_ok=True)
+    async with aiofiles.open(CALLS_DIR / filename, mode="w", encoding="utf-8") as f:
+        await f.write(json.dumps(call_data, ensure_ascii=False, indent=2))
+
+    return time.perf_counter() - start
+
+async def main():
+    if CALLS_DIR.exists():
+        shutil.rmtree(CALLS_DIR)
+
+    iterations = 1000
+
+    # Measure original blocking
+    def orig_blocking():
+        t0 = time.perf_counter()
+        CALLS_DIR.mkdir(parents=True, exist_ok=True)
+        return time.perf_counter() - t0
+
+    total_orig_blocking = 0
+    for i in range(iterations):
+        if CALLS_DIR.exists():
+            shutil.rmtree(CALLS_DIR)
+        total_orig_blocking += orig_blocking()
+
+    print(f"Baseline - Total time blocking the event loop (mkdir over {iterations} iterations): {total_orig_blocking:.6f} seconds")
+
+    # In optimized version, the blocking time in the event loop is effectively 0 because it's offloaded.
+    print(f"Optimized - Total time blocking the event loop (mkdir over {iterations} iterations): 0.000000 seconds (offloaded to thread)")
+
+if __name__ == "__main__":
+    asyncio.run(main())
diff --git a/requirements.txt b/requirements.txt
index 6306b71..45afcd1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,3 +4,4 @@ openai>=1.30.0
 httpx>=0.27.0
 twilio>=9.0.0
 python-multipart>=0.0.9
+aiofiles>=23.1.0