diff --git a/src/sync/backends/jmap/contacts.ts b/src/sync/backends/jmap/contacts.ts
index a31017b..990cd56 100644
--- a/src/sync/backends/jmap/contacts.ts
+++ b/src/sync/backends/jmap/contacts.ts
@@ -673,6 +673,29 @@ export async function reconcileContacts({ transport, account, handlers, useWebSo
   return syncContacts({ transport, account, handlers, useWebSocket });
 }
 
+/**
+ * Reconcile the local cache for a small, known set of cards after a
+ * single-contact mutation (whitelist, add, edit), instead of re-pulling
+ * the entire address book the way `reconcileContacts` does. Cost is
+ * O(books) + O(ids) rather than O(all contacts), so a whitelist stays
+ * fast no matter how many contacts the account has.
+ *
+ * Address books are synced first (they are few) so a card filed in a
+ * book that was created in the same operation — e.g. "Trusted senders" —
+ * resolves locally; then only the named card ids are fetched + upserted.
+ */
+export async function reconcileContactCards({
+  transport, account, handlers, ids, useWebSocket = false,
+}) {
+  await syncAddressBooks({ transport, account, handlers, useWebSocket });
+  const list = (ids ?? []).filter(Boolean);
+  if (list.length === 0) return { fetched: 0 };
+  const fetched = await fetchAndPersistContactCards({
+    transport, account, handlers, ids: list, useWebSocket,
+  });
+  return { fetched };
+}
+
 /**
  * Resolve the remote id of the account's primary address book for new
  * contacts: the one flagged `isDefault`, else the first book that is
diff --git a/src/sync/backends/jmap/outbox.ts b/src/sync/backends/jmap/outbox.ts
index bd4896d..ebf57db 100644
--- a/src/sync/backends/jmap/outbox.ts
+++ b/src/sync/backends/jmap/outbox.ts
@@ -52,7 +52,7 @@ import {
   createContactCard,
   updateContactCard,
   deleteContactCard,
-  reconcileContacts,
+  reconcileContactCards,
 } from './contacts';
 import { base64ToBytes, extractDataUriImages } from '../../../utils/inline-images';
 
@@ -195,11 +195,12 @@ async function runWhitelistSender({ transport, account, handlers, request, useWe
   if (!result.ok) {
     return { ok: false, error: result.error ?? { type: 'serverFail' } };
   }
-  // Pull the new cards (and their book) into the local cache once, after
-  // all trusted cards exist, so they show up in the contacts view without
-  // waiting for a StateChange push. Best-effort: the trust already took
+  // Pull only the newly-trusted card(s) into the local cache so they show
+  // up in the contacts view without waiting for a StateChange push.
+  // Targeted (not a full address-book resync) so a whitelist stays fast
+  // regardless of contact count. Best-effort: the trust already took
   // effect server-side, so a reconcile failure must not fail the mutation.
-  await reconcileContactsQuietly({ transport, account, handlers, useWebSocket });
+  await reconcileContactCardsQuietly({ transport, account, handlers, ids: result.ids, useWebSocket });
   return { ok: true };
 }
 
@@ -224,7 +225,9 @@ async function runCreateContact({ transport, account, handlers, request, useWebS
   if (!result.ok) {
     return { ok: false, error: result.error ?? { type: 'serverFail' } };
   }
-  await reconcileContactsQuietly({ transport, account, handlers, useWebSocket });
+  await reconcileContactCardsQuietly({
+    transport, account, handlers, ids: result.id ? [result.id] : [], useWebSocket,
+  });
   return { ok: true };
 }
 
@@ -245,7 +248,9 @@ async function runUpdateContact({ transport, account, handlers, request, useWebS
   if (!result.ok) {
     return { ok: false, error: result.error ?? { type: 'serverFail' } };
   }
-  await reconcileContactsQuietly({ transport, account, handlers, useWebSocket });
+  await reconcileContactCardsQuietly({
+    transport, account, handlers, ids: request?.remoteId ? [request.remoteId] : [], useWebSocket,
+  });
   return { ok: true };
 }
 
@@ -272,9 +277,9 @@ async function runDeleteContact({ transport, account, handlers, request, useWebS
   return { ok: true };
 }
 
-async function reconcileContactsQuietly({ transport, account, handlers, useWebSocket }) {
+async function reconcileContactCardsQuietly({ transport, account, handlers, ids, useWebSocket }) {
   try {
-    await reconcileContacts({ transport, account, handlers, useWebSocket });
+    await reconcileContactCards({ transport, account, handlers, ids, useWebSocket });
   } catch {
     // Cache will catch up on the next StateChange push or periodic sync.
   }
diff --git a/tests/unit/sync/jmap-contacts.test.ts b/tests/unit/sync/jmap-contacts.test.ts
index b268553..9693ad7 100644
--- a/tests/unit/sync/jmap-contacts.test.ts
+++ b/tests/unit/sync/jmap-contacts.test.ts
@@ -13,8 +13,19 @@ import {
   updateContactCard,
   deleteContactCard,
 } from '../../../src/sync/backends/jmap/contacts';
+import { processMutationRow } from '../../../src/sync/backends/jmap/outbox';
 import { MockTransport } from './_mock-transport';
 
+function jmapCalls(transport, method) {
+  const out = [];
+  for (const req of transport.requests) {
+    for (const [m, params] of req.methodCalls) {
+      if (m === method) out.push(params);
+    }
+  }
+  return out;
+}
+
 function countMethod(transport, name) {
   return transport.requests.filter(
     (r) => r.methodCalls.some(([m]) => m === name),
@@ -507,3 +518,121 @@ describe('syncContactCardChanges', () => {
     expect(Number(destroyed.is_deleted)).toBe(1);
   });
 });
+
+describe('whitelist reconcile cost is independent of contact count', () => {
+  async function seedLocalContacts(n) {
+    await handlers[DB_RPC.ADDRESSBOOK_UPSERT_MANY]({
+      accountId: account.id,
+      serviceKind: SERVICE_KIND.JMAP_CONTACTS,
+      addressbooks: [
+        { remoteId: 'book-default', name: 'Default', isDefault: true },
+        { remoteId: 'book-trusted', name: 'Trusted senders', isDefault: false },
+      ],
+    });
+    const books = await handlers[DB_RPC.ADDRESSBOOK_LIST]({ accountId: account.id });
+    const defaultLocal = books.find((b) => b.remote_id === 'book-default').id;
+    const contacts = Array.from({ length: n }, (_, i) => ({
+      addressbookId: defaultLocal,
+      remoteId: `seed-${i}`,
+      uid: null,
+      etag: null,
+      fullName: `Seed ${i}`,
+      displayName: `Seed ${i}`,
+      givenName: null,
+      familyName: null,
+      organization: null,
+      vcardText: null,
+      vcardVersion: null,
+      rawJson: '{}',
+      isDeleted: false,
+      emails: [{ position: 0, email: `seed-${i}@x.invalid`, label: null, isPreferred: true }],
+    }));
+    await handlers[DB_RPC.CONTACT_UPSERT_MANY]({ accountId: account.id, contacts });
+  }
+
+  function countContacts() {
+    return engine.get(
+      'SELECT COUNT(*) AS n FROM contacts WHERE account_id = ? AND is_deleted = 0',
+      [account.id],
+    );
+  }
+
+  it('batch-whitelisting 200 mails from 150 senders on a 1099-contact book fetches only the 150 new cards', async () => {
+    await seedLocalContacts(1099);
+    expect((await countContacts()).n).toBe(1099);
+
+    // 200 junk messages multi-selected, from 150 distinct senders (50
+    // repeats), as the store's whitelistSenders would hand them off.
+    const senders = Array.from({ length: 200 }, (_, i) => ({
+      email: `batch-${i % 150}@junk.invalid`,
+      name: `Sender ${i % 150}`,
+    }));
+    expect(new Set(senders.map((s) => s.email)).size).toBe(150);
+
+    // The "server" holds 1099 cards, but the batch whitelist must never
+    // pull them: a full ContactCard/query (position/limit) would be the
+    // old O(contacts) reconcile; the existence check uses a {email}
+    // (OR-of-emails) filter.
+    const transport = new MockTransport();
+    let fullListQueried = false;
+    let setCalls = 0;
+    let createdInOneCall = 0;
+    transport.handle('ContactCard/query', (params) => {
+      if (params.position != null || params.limit != null) {
+        fullListQueried = true;
+        return { ids: Array.from({ length: 1099 }, (_, i) => `seed-${i}`), total: 1099, state: 's' };
+      }
+      return { ids: [], total: 0 }; // existence check: none of the senders carded yet
+    });
+    transport.handle('AddressBook/get', () => ({
+      list: [
+        { id: 'book-default', name: 'Default', isDefault: true },
+        { id: 'book-trusted', name: 'Trusted senders', isDefault: false },
+      ],
+    }));
+    transport.handle('ContactCard/set', (params) => {
+      setCalls += 1;
+      const created = {};
+      const keys = Object.keys(params.create ?? {});
+      createdInOneCall = Math.max(createdInOneCall, keys.length);
+      for (const key of keys) created[key] = { id: key }; // distinct ids
+      return { created };
+    });
+    transport.handle('ContactCard/get', (params) => ({
+      list: (params.ids ?? []).map((id) => ({
+        '@type': 'Card',
+        id,
+        kind: 'individual',
+        name: { full: `Trusted ${id}` },
+        emails: { e1: { '@type': 'EmailAddress', address: `wl-${id}@junk.invalid` } },
+        addressBookIds: { 'book-trusted': true },
+      })),
+    }));
+
+    const result = await processMutationRow({
+      transport,
+      account,
+      handlers,
+      row: {
+        mutation_type: 'whitelistSender',
+        request_json: JSON.stringify({ senders }),
+      },
+    });
+    expect(result.ok).toBe(true);
+
+    // All 150 unique senders trusted in a single batched ContactCard/set
+    // (200 mails de-duped to 150 cards, one call, not a per-sender loop).
+    expect(setCalls).toBe(1);
+    expect(createdInOneCall).toBe(150);
+
+    // The 1099-contact book is never re-pulled...
+    expect(fullListQueried).toBe(false);
+    // ...only the 150 new trusted cards are fetched.
+    const idsFetched = jmapCalls(transport, 'ContactCard/get')
+      .reduce((sum, p) => sum + (p.ids?.length ?? 0), 0);
+    expect(idsFetched).toBe(150);
+
+    // The 1099 existing contacts are untouched; exactly 150 were added.
+    expect((await countContacts()).n).toBe(1249);
+  });
+});