diff --git a/src/screens/ContactsScreen.tsx b/src/screens/ContactsScreen.tsx
index 74eb72ec..140c02ab 100644
--- a/src/screens/ContactsScreen.tsx
+++ b/src/screens/ContactsScreen.tsx
@@ -50,8 +50,8 @@ interface Section {
   data: ContactCard[];
 }
 
-function groupContacts(contacts: ContactCard[], byLastName: boolean): Section[] {
-  const sorted = sortContactsByName(contacts, byLastName);
+/** Letter sections for contacts already in `sortContactsByName` order. */
+function groupContacts(sorted: ContactCard[], byLastName: boolean): Section[] {
   const groups: Record<string, ContactCard[]> = {};
   for (const c of sorted) {
     // Sections follow the sort name, so with "sort by last name" Alice Smith
@@ -137,7 +137,7 @@ export default function ContactsScreen() {
   const groups = React.useMemo(() => contacts.filter(isGroup), [contacts]);
   const individuals = React.useMemo(() => contacts.filter((c) => !isGroup(c)), [contacts]);
 
-  const visible = React.useMemo(() => {
+  const categoryContacts = React.useMemo(() => {
     let filtered: ContactCard[];
     switch (selectedCategory.type) {
       case 'all':
@@ -162,14 +162,25 @@ export default function ContactsScreen() {
       default:
         filtered = individuals;
     }
-    if (!searchQuery) return filtered;
-    return filtered.filter((c) => matchesContactSearch(c, searchQuery));
-  }, [contacts, individuals, selectedCategory, searchQuery]);
+    return filtered;
+  }, [contacts, individuals, selectedCategory]);
+
+  // Sort once per list or order change; search only filters the sorted list,
+  // so typing never re-sorts the address book (PF10).
+  const sorted = React.useMemo(
+    () => sortContactsByName(categoryContacts, sortByLastName),
+    [categoryContacts, sortByLastName],
+  );
+
+  const visible = React.useMemo(() => {
+    if (!searchQuery) return sorted;
+    return sorted.filter((c) => matchesContactSearch(c, searchQuery));
+  }, [sorted, searchQuery]);
 
   const sections = React.useMemo<Section[]>(() => {
     if (groupByLetter) return groupContacts(visible, sortByLastName);
     // Flat list - single unnamed section keeps SectionList rendering simple.
-    return [{ title: '', data: sortContactsByName(visible, sortByLastName) }];
+    return [{ title: '', data: visible }];
   }, [visible, groupByLetter, sortByLastName]);
 
   const selectionMode = selection.size > 0;
diff --git a/src/stores/__tests__/contacts-store.test.ts b/src/stores/__tests__/contacts-store.test.ts
index 268452fc..092d3c7f 100644
--- a/src/stores/__tests__/contacts-store.test.ts
+++ b/src/stores/__tests__/contacts-store.test.ts
@@ -526,4 +526,10 @@ describe('sortContactsByName (#963)', () => {
     sortContactsByName(input, true);
     expect(input.map((c) => c.id)).toEqual(['zoe', 'alice', 'bob', 'acme']);
   });
+
+  it('compares case- and accent-insensitively, keeping ties in list order (PF10)', () => {
+    const named = (id: string, full: string) => card(id, { name: { full } });
+    const list = [named('z', 'Zed'), named('e2', 'emile'), named('e1', 'Émile'), named('a', 'Anna')];
+    expect(sortContactsByName(list, false).map((c) => c.id)).toEqual(['a', 'e2', 'e1', 'z']);
+  });
 });
diff --git a/src/stores/contacts-store.ts b/src/stores/contacts-store.ts
index 7ae71d83..6045d941 100644
--- a/src/stores/contacts-store.ts
+++ b/src/stores/contacts-store.ts
@@ -928,9 +928,14 @@ export function sortContactsByDisplayName(contacts: ContactCard[]): ContactCard[
   return sortContactsByName(contacts, false);
 }
 
+// One collator for every contact sort: `localeCompare` with options sets one
+// up per comparison, which made a large address book slow to sort (PF10).
+const contactNameCollator = new Intl.Collator(undefined, { sensitivity: 'base' });
+
 /** Contact list order: by display name, or surname first with `byLastName` (#963). */
 export function sortContactsByName(contacts: ContactCard[], byLastName: boolean): ContactCard[] {
-  return [...contacts].sort((a, b) =>
-    getContactSortName(a, byLastName).localeCompare(getContactSortName(b, byLastName), undefined, { sensitivity: 'base' }),
-  );
+  // Work out each sort name once instead of twice per comparison.
+  const keyed = contacts.map((contact) => ({ contact, key: getContactSortName(contact, byLastName) }));
+  keyed.sort((a, b) => contactNameCollator.compare(a.key, b.key));
+  return keyed.map((entry) => entry.contact);
 }