diff --git a/lib/Migration/Version8000Date20260603120000.php b/lib/Migration/Version8000Date20260603120000.php
new file mode 100644
index 000000000..b5fc71e65
--- /dev/null
+++ b/lib/Migration/Version8000Date20260603120000.php
@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Activity\Migration;
+
+use Closure;
+use OCP\DB\ISchemaWrapper;
+use OCP\Migration\IOutput;
+use OCP\Migration\SimpleMigrationStep;
+
+/**
+ * Widen the `amq_affecteduser` index on `activity_mq` to also cover
+ * `amq_latest_send`.
+ *
+ * The email digest cron (`MailQueueHandler::getAffectedUsers()`) runs on every
+ * cron tick and does
+ *   SELECT amq_affecteduser, MIN(amq_latest_send)
+ *   FROM activity_mq WHERE amq_latest_send < ? GROUP BY amq_affecteduser
+ * The previous `amp_user` index only contained `amq_affecteduser`, so the
+ * aggregate forced a full index scan plus a temporary table to resolve the
+ * GROUP BY. Including `amq_latest_send` lets MariaDB/MySQL resolve the MIN()
+ * with a loose index scan ("Using index for group-by"), reading one entry per
+ * user instead of the whole queue.
+ *
+ * `amp_user` is a strict prefix of the new index, so it becomes redundant and
+ * is dropped to avoid carrying two overlapping indexes.
+ */
+class Version8000Date20260603120000 extends SimpleMigrationStep {
+	/**
+	 * @param IOutput $output
+	 * @param Closure $schemaClosure The `\Closure` returns a `ISchemaWrapper`
+	 * @param array $options
+	 * @return null|ISchemaWrapper
+	 */
+	#[\Override]
+	public function changeSchema(IOutput $output, Closure $schemaClosure, array $options): ?ISchemaWrapper {
+		/** @var ISchemaWrapper $schema */
+		$schema = $schemaClosure();
+
+		if (!$schema->hasTable('activity_mq')) {
+			return null;
+		}
+
+		$table = $schema->getTable('activity_mq');
+
+		if (!$table->hasIndex('amp_user_send')) {
+			$table->addIndex(['amq_affecteduser', 'amq_latest_send'], 'amp_user_send');
+		}
+
+		if ($table->hasIndex('amp_user')) {
+			$table->dropIndex('amp_user');
+		}
+
+		return $schema;
+	}
+}
diff --git a/tests/Migration/Version8000Date20260603120000Test.php b/tests/Migration/Version8000Date20260603120000Test.php
new file mode 100644
index 000000000..969d3449c
--- /dev/null
+++ b/tests/Migration/Version8000Date20260603120000Test.php
@@ -0,0 +1,105 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Activity\Tests\Migration;
+
+use Doctrine\DBAL\Schema\Table;
+use OCA\Activity\Migration\Version8000Date20260603120000;
+use OCA\Activity\Tests\TestCase;
+use OCP\DB\ISchemaWrapper;
+use OCP\Migration\IOutput;
+use PHPUnit\Framework\MockObject\MockObject;
+
+/**
+ * @see Version8000Date20260603120000
+ */
+class Version8000Date20260603120000Test extends TestCase {
+	protected IOutput&MockObject $output;
+	protected Version8000Date20260603120000 $migration;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->output = $this->createMock(IOutput::class);
+		$this->migration = new Version8000Date20260603120000();
+	}
+
+	/**
+	 * On a queue that still has the legacy single-column `amp_user` index the
+	 * migration adds the covering composite index and drops the now-redundant one.
+	 */
+	public function testAddsCompositeIndexAndDropsRedundantOne(): void {
+		$table = $this->createMock(Table::class);
+		$table->method('hasIndex')
+			->willReturnMap([
+				['amp_user_send', false],
+				['amp_user', true],
+			]);
+		$table->expects($this->once())
+			->method('addIndex')
+			->with(['amq_affecteduser', 'amq_latest_send'], 'amp_user_send');
+		$table->expects($this->once())
+			->method('dropIndex')
+			->with('amp_user');
+
+		$schema = $this->getSchemaMock($table);
+
+		$result = $this->migration->changeSchema($this->output, fn (): ISchemaWrapper => $schema, []);
+		$this->assertSame($schema, $result, 'The schema must be returned when it was changed');
+	}
+
+	/**
+	 * Running the migration again (composite index present, legacy index gone)
+	 * must be a no-op so re-runs / fresh installs are not touched.
+	 */
+	public function testIsIdempotentWhenAlreadyMigrated(): void {
+		$table = $this->createMock(Table::class);
+		$table->method('hasIndex')
+			->willReturnMap([
+				['amp_user_send', true],
+				['amp_user', false],
+			]);
+		$table->expects($this->never())
+			->method('addIndex');
+		$table->expects($this->never())
+			->method('dropIndex');
+
+		$schema = $this->getSchemaMock($table);
+
+		$result = $this->migration->changeSchema($this->output, fn (): ISchemaWrapper => $schema, []);
+		$this->assertSame($schema, $result, 'Re-running must not add or drop the indexes again');
+	}
+
+	/**
+	 * The mail queue table does not exist on every install (e.g. before its
+	 * creation migration ran), so a missing table must be skipped gracefully.
+	 */
+	public function testSkipsWhenTableIsMissing(): void {
+		$schema = $this->createMock(ISchemaWrapper::class);
+		$schema->method('hasTable')
+			->with('activity_mq')
+			->willReturn(false);
+		$schema->expects($this->never())
+			->method('getTable');
+
+		$result = $this->migration->changeSchema($this->output, fn (): ISchemaWrapper => $schema, []);
+		$this->assertNull($result);
+	}
+
+	protected function getSchemaMock(Table&MockObject $table): ISchemaWrapper&MockObject {
+		$schema = $this->createMock(ISchemaWrapper::class);
+		$schema->method('hasTable')
+			->with('activity_mq')
+			->willReturn(true);
+		$schema->method('getTable')
+			->with('activity_mq')
+			->willReturn($table);
+
+		return $schema;
+	}
+}