diff --git a/accounts/api_accounts.py b/accounts/api_accounts.py
index d9ddfff..9a5cf2f 100644
--- a/accounts/api_accounts.py
+++ b/accounts/api_accounts.py
@@ -91,7 +91,10 @@ async def create_subscription(
     if not label:
         # Cache-only lookup — never block the event loop on Nominatim. The frontend
         # normally supplies the label from /api/v2/place; this is just a fallback.
-        _, label = climate.reverse_geocode_cached(cell["center_lat"], cell["center_lon"])
+        # Still a sync DB read under the hood, so keep it off the event loop too.
+        _, label = await run_in_threadpool(
+            climate.reverse_geocode_cached, cell["center_lat"], cell["center_lon"]
+        )
 
     sub = Subscription(
         user_id=user.id,
diff --git a/accounts/users.py b/accounts/users.py
index 80a1e16..a08fb46 100644
--- a/accounts/users.py
+++ b/accounts/users.py
@@ -9,6 +9,7 @@ import secrets
 import uuid
 
 from fastapi import Depends, Request
+from fastapi.concurrency import run_in_threadpool
 from fastapi_users import BaseUserManager, FastAPIUsers, InvalidPasswordException, UUIDIDMixin
 from fastapi_users.authentication import AuthenticationBackend, CookieTransport
 from fastapi_users.authentication.strategy.db import DatabaseStrategy
@@ -89,7 +90,10 @@ class UserManager(UUIDIDMixin, BaseUserManager[User, uuid.UUID]):
         if not mailer.enabled():
             return
         link = f"{BASE_URL}{BASE}/?verify_token={token}"
-        mailer.send(
+        # mailer.send() opens a blocking SMTP connection — fires on every
+        # register/resend-verification request, so keep it off the event loop.
+        await run_in_threadpool(
+            mailer.send,
             user.email,
             "Confirm your Thermograph account",
             "Welcome to Thermograph! Confirm your email address to finish setting up "
diff --git a/notifications/digest.py b/notifications/digest.py
index 9e557d7..9ac5433 100644
--- a/notifications/digest.py
+++ b/notifications/digest.py
@@ -13,6 +13,7 @@ import re
 import time
 
 from fastapi import Request, Response
+from fastapi.concurrency import run_in_threadpool
 from fastapi.responses import JSONResponse
 from sqlalchemy import select
 
@@ -67,7 +68,9 @@ async def _store(email: str, place: str | None, source: str | None) -> None:
     # Nothing is sent until a real mail backend is configured; the console
     # backend logs it so the flow can be verified end-to-end without a server.
     if mailer.enabled():
-        mailer.send(
+        # Blocking SMTP connect+send — keep it off the event loop.
+        await run_in_threadpool(
+            mailer.send,
             email,
             "Confirm your Thermograph digest",
             "Thanks for signing up for the Thermograph monthly digest.\n\n"
@@ -103,7 +106,8 @@ async def signup(request: Request) -> Response:
             await _store(email, place, source)
         except Exception:  # noqa: BLE001 - never surface storage detail to the form
             pass
-        metrics.record_event(
+        await run_in_threadpool(
+            metrics.record_event,
             "home.digest_signup",
             referer=request.headers.get("referer"),
             host=request.headers.get("host"),
diff --git a/web/app.py b/web/app.py
index 35f664e..29c1062 100644
--- a/web/app.py
+++ b/web/app.py
@@ -12,6 +12,7 @@ import time
 
 import httpx
 from fastapi import APIRouter, FastAPI, HTTPException, Query, Request, Response
+from fastapi.concurrency import run_in_threadpool
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.middleware.gzip import GZipMiddleware
 from fastapi.responses import RedirectResponse
@@ -249,12 +250,16 @@ async def revalidate_static(request, call_next):
     path = request.url.path
     try:
         cat = metrics.classify_inbound(path, BASE)
-        metrics.record_inbound(cat, response.status_code)
+        # Both are blocking (sync DB write, sync file write) — run off the event
+        # loop so one request's instrumentation can't stall every other request
+        # this worker is serving concurrently.
+        await run_in_threadpool(metrics.record_inbound, cat, response.status_code)
         # Retain per-request client IPs for later analysis; skip static assets and the
         # dashboard's own metrics polling to keep the log to real, meaningful traffic.
         if cat not in ("static", "metrics", "event"):
-            audit.log_access({"ip": _client_ip(request), "method": request.method,
-                              "path": path, "status": response.status_code, "cat": cat})
+            await run_in_threadpool(audit.log_access, {
+                "ip": _client_ip(request), "method": request.method,
+                "path": path, "status": response.status_code, "cat": cat})
     except Exception:  # noqa: BLE001 - never let instrumentation break a response
         pass
     pages = (BASE, f"{BASE}/", f"{BASE}/calendar", f"{BASE}/day", f"{BASE}/score",
@@ -724,7 +729,8 @@ async def api_event(request: Request) -> Response:
     except Exception:  # noqa: BLE001 - a junk body is just a no-op event
         pass
     if name:
-        metrics.record_event(
+        await run_in_threadpool(
+            metrics.record_event,
             name,
             referer=request.headers.get("referer"),
             host=request.headers.get("host"),
@@ -810,7 +816,10 @@ async def discord_interactions(request: Request) -> Response:
     """Discord posts each slash-command interaction here. Verification runs over the
     RAW body, so this must read bytes, not request.json()."""
     raw = await request.body()
-    status, payload = discord_interactions_mod.handle(
+    # handle() does real work for /grade (sync parquet/DB reads + grading math),
+    # not just the cheap signature check — keep it off the event loop.
+    status, payload = await run_in_threadpool(
+        discord_interactions_mod.handle,
         raw,
         request.headers.get("x-signature-ed25519", ""),
         request.headers.get("x-signature-timestamp", ""),