diff --git a/apps/backend/prisma/migrations/20250831003652_session_id_event_indices/migration.sql b/apps/backend/prisma/migrations/20250831003652_session_id_event_indices/migration.sql
new file mode 100644
index 000000000..9d37391d9
--- /dev/null
+++ b/apps/backend/prisma/migrations/20250831003652_session_id_event_indices/migration.sql
@@ -0,0 +1,6 @@
+-- It's very common to query by sessionId, userId, projectId, branchId, and eventStartedAt at the same time.
+-- We can use a composite index to speed up the query.
+-- Sadly we can't add this to the Prisma schema itself because Prisma does not understand composite indexes of JSONB fields.
+-- So we have to add it manually.
+-- (This is similar to the older idx_event_userid_projectid_branchid_eventstartedat index, but with sessionId added.)
+CREATE INDEX idx_event_sessionid_userid_projectid_branchid_eventstartedat ON "Event" ((data->>'projectId'), (data->>'branchId'), (data->>'userId'), (data->>'sessionId'), "eventStartedAt");
diff --git a/apps/backend/src/app/api/latest/auth/sessions/crud.tsx b/apps/backend/src/app/api/latest/auth/sessions/crud.tsx
index 94ca99228..e20f8df93 100644
--- a/apps/backend/src/app/api/latest/auth/sessions/crud.tsx
+++ b/apps/backend/src/app/api/latest/auth/sessions/crud.tsx
@@ -49,6 +49,9 @@ export const sessionsCrudHandlers = createLazyProxy(() => createCrudHandlers(ses
           ? Prisma.sql`data->>'sessionId' = ANY(${Prisma.sql`ARRAY[${Prisma.join(refreshTokenObjs.map(s => s.id))}]`})`
           : Prisma.sql`FALSE`}
         AND "systemEventTypeIds" @> '{"$session-activity"}'
+        AND data->>'userId' = ${query.user_id}
+        AND data->>'projectId' = ${auth.tenancy.project.id}
+        AND COALESCE(data->>'branchId', 'main') = ${auth.tenancy.branchId}
         GROUP BY data->>'sessionId'
       )
       SELECT e.data->>'sessionId' as "sessionId", 
diff --git a/apps/backend/src/app/api/latest/users/crud.tsx b/apps/backend/src/app/api/latest/users/crud.tsx
index 1a9e94d41..9a26dbb08 100644
--- a/apps/backend/src/app/api/latest/users/crud.tsx
+++ b/apps/backend/src/app/api/latest/users/crud.tsx
@@ -210,7 +210,10 @@ export const getUsersLastActiveAtMillis = async (projectId: string, branchId: st
   const events = await prisma.$queryRaw<Array<{ userId: string, lastActiveAt: Date }>>`
     SELECT data->>'userId' as "userId", MAX("eventStartedAt") as "lastActiveAt"
     FROM ${sqlQuoteIdent(schema)}."Event"
-    WHERE data->>'userId' = ANY(${Prisma.sql`ARRAY[${Prisma.join(userIds)}]`}) AND data->>'projectId' = ${projectId} AND COALESCE("data"->>'branchId', 'main') = ${branchId} AND "systemEventTypeIds" @> '{"$user-activity"}'
+    WHERE data->>'userId' = ANY(${Prisma.sql`ARRAY[${Prisma.join(userIds)}]`})
+      AND data->>'projectId' = ${projectId}
+      AND COALESCE("data"->>'branchId', 'main') = ${branchId}
+      AND "systemEventTypeIds" @> '{"$user-activity"}'
     GROUP BY data->>'userId'
   `;