diff --git a/src/database/mongo.js b/src/database/mongo.js
index 310867cef5..4e2ba46a66 100644
--- a/src/database/mongo.js
+++ b/src/database/mongo.js
@@ -86,6 +86,9 @@ mongoModule.createIndices = async function () {
 	const collection = mongoModule.client.collection('objects');
 	await collection.createIndex({ _key: 1, score: -1 }, { background: true });
 	await collection.createIndex({ _key: 1, value: -1 }, { background: true, unique: true, sparse: true });
+	await collection.createIndex(
+		{ members: 1, _key: 1}, { background: true, partialFilterExpression: { members: { $exists: true } } }
+	);
 	await collection.createIndex({ expireAt: 1 }, { expireAfterSeconds: 0, background: true });
 	winston.info('[database] Checking database indices done!');
 };
diff --git a/src/database/mongo/sets.js b/src/database/mongo/sets.js
index 784b6acd70..66b4baebab 100644
--- a/src/database/mongo/sets.js
+++ b/src/database/mongo/sets.js
@@ -115,7 +115,7 @@ module.exports = function (module) {
 
 		await module.client.collection('objects').bulkWrite([
 			{ updateMany: { filter: { _key: filterKey }, update: update } },
-			{ deleteMany: { filter: { _key: filterKey, members: { $size: 0 } } } },
+			{ deleteMany: { filter: { _key: filterKey, members: [] } } },
 		], { ordered: true });
 	}
 
@@ -128,8 +128,9 @@ module.exports = function (module) {
 		const item = await module.client.collection('objects').findOne({
 			_key: key, members: value,
 		}, {
-			projection: { _id: 0, members: 0 },
+			projection: { _id: 0, _key: 1},
 		});
+
 		return item !== null && item !== undefined;
 	};
 
@@ -137,15 +138,31 @@ module.exports = function (module) {
 		if (!key || !Array.isArray(values) || !values.length) {
 			return [];
 		}
-		values = values.map(v => helpers.valueToString(v));
+		const stringValues = values.map(v => helpers.valueToString(v));
 
-		const result = await module.client.collection('objects').findOne({
-			_key: key,
-		}, {
-			projection: { _id: 0, _key: 0 },
-		});
-		const membersSet = new Set(result && Array.isArray(result.members) ? result.members : []);
-		return values.map(v => membersSet.has(v));
+		const pipeline = [
+			{
+				$match: {
+					_key: key,
+					members: { $in: stringValues, $exists: true }, // Trigger the partial index
+				},
+			},
+			{
+				$project: {
+					_id: 0,
+					results: {
+						$map: {
+							input: stringValues,
+							as: 'val',
+							in: { $in: ['$$val', '$members'] },
+						},
+					},
+				},
+			},
+		];
+
+		const [doc] = await module.client.collection('objects').aggregate(pipeline).toArray();
+		return doc ? doc.results : values.map(() => false);
 	};
 
 	module.isMemberOfSets = async function (sets, value) {
@@ -157,7 +174,7 @@ module.exports = function (module) {
 		const result = await module.client.collection('objects').find({
 			_key: { $in: sets }, members: value,
 		}, {
-			projection: { _id: 0, members: 0 },
+			projection: { _id: 0, _key: 1 },
 		}).toArray();
 
 		const map = {};
@@ -176,7 +193,7 @@ module.exports = function (module) {
 		const data = await module.client.collection('objects').findOne({
 			_key: key,
 		}, {
-			projection: { _id: 0, _key: 0 },
+			projection: { _id: 0, members: 1 },
 		});
 		return data ? data.members : [];
 	};
diff --git a/src/database/mongo/sorted.js b/src/database/mongo/sorted.js
index eceea5e342..c6f1ee90ce 100644
--- a/src/database/mongo/sorted.js
+++ b/src/database/mongo/sorted.js
@@ -393,13 +393,13 @@ module.exports = function (module) {
 			return [];
 		}
 		const arrayOfKeys = keys.length > 1;
-		const projection = { _id: 0, value: 1 };
+		const projection = arrayOfKeys ?
+			{ _id: 0, _key: 1, value: 1} :
+			{ _id: 0, value: 1 };
 		if (withScores) {
 			projection.score = 1;
 		}
-		if (arrayOfKeys) {
-			projection._key = 1;
-		}
+
 		const data = await module.client.collection('objects').find({
 			_key: arrayOfKeys ? { $in: keys } : keys[0],
 		}, { projection: projection })
diff --git a/src/groups/invite.js b/src/groups/invite.js
index 747111d156..e5b7c7a1ca 100644
--- a/src/groups/invite.js
+++ b/src/groups/invite.js
@@ -54,9 +54,15 @@ module.exports = function (Groups) {
 		if (!Array.isArray(groupNames)) {
 			groupNames = [groupNames];
 		}
-		const sets = [];
-		groupNames.forEach(groupName => sets.push(`group:${groupName}:pending`, `group:${groupName}:invited`));
-		await db.setsRemove(sets, uid);
+		const sets = [
+			...groupNames.map(g => `group:${g}:pending`),
+			...groupNames.map(g => `group:${g}:invited`),
+		];
+		const isMembers = await db.isMemberOfSets(sets, uid);
+		const toRemoveSets = sets.filter((set, index) => isMembers[index]);
+		if (toRemoveSets.length) {
+			await db.setsRemove(toRemoveSets, uid);
+		}
 	};
 
 	Groups.invite = async function (groupName, uids) {
diff --git a/src/upgrades/4.10.0/mongodb-set-members-index.js b/src/upgrades/4.10.0/mongodb-set-members-index.js
new file mode 100644
index 0000000000..f7a2284eb7
--- /dev/null
+++ b/src/upgrades/4.10.0/mongodb-set-members-index.js
@@ -0,0 +1,22 @@
+'use strict';
+
+
+const db = module.parent.require('./database');
+
+module.exports = {
+	name: 'Add a partial index on set members',
+	timestamp: Date.UTC(2026, 2, 11),
+	method: async function () {
+		const nconf = require.main.require('nconf');
+		const isMongo = nconf.get('database') === 'mongo';
+		if (!isMongo) {
+			return;
+		}
+
+		await db.client.collection('objects').createIndex(
+			{ members: 1, _key: 1 },
+			{ background: true, partialFilterExpression: { members: { $exists: true } } },
+		);
+	},
+};
+