Flutter notifications from firebase messaging is delayed?

sometime the notification is delayed for no reason and then comes all together iam using https://pub.dev/packages/flutter_local_notifications https://pub.dev/packages/firebase_messaging

here is my code from cloud functions ( nodejs )

exports.messageTrigger = functions.firestore.document('/Messages/{messageID}').onCreate(

  
  async (snapshot, context) => {
    function capitalizeFirstLetter(string) {
      return string.charAt(0).toUpperCase() + string.slice(1);
    }
    var currentRoomUsers = snapshot.data().members;

    
    currentRoomUsers.forEach(  (userID) => {
        db.collection('Users').doc(userID).get().then(async(doc)=>{
        if(doc.exists && doc.id != snapshot.data().senderID){
          const message = {
           
            data: {
            
              title: `New message from ${capitalizeFirstLetter(snapshot.data().room)}`,
              body:  snapshot.data().type == 'm.text' ? 'Sent a new message' : snapshot.data().type == 'm.start'? 'Invited you for a call' : snapshot.data().type == 'm.image' ? 'Sent an image' : 'Sent a new message'
            },
            tokens: doc.data()['Device_token'],
            android: {
              priority: 'high',
            },
            priority: "high",
          }

      await admin.messaging().sendMulticast(message);
      
        }else {
          console.log("No such document!");
        }
      }).catch((error)=>{
        console.log("Error getting document:", error);
      });
    
    
    
     
    }
      
      );

 }
);

Firestore documents are fetched asynchronously and the sendMulticast() statement here may run before all the documents are fetched. Also, you must terminate Cloud Functions by returning a Promise. Since you are using an async function, try refactoring the code with async-await syntax as shown below:

exports.messageTrigger = functions.firestore
  .document("/Messages/{messageID}")
  .onCreate(async (snapshot, context) => {
    try {
      function capitalizeFirstLetter(string) {
        return string.charAt(0).toUpperCase() + string.slice(1);
      }

      const currentRoomUsers = snapshot.data().members;

      // use Promise.all() to fetch all documents 
      const customRoomUsersSnap = await Promise.all(
        currentRoomUsers.map((user) =>
          db.collection("Users").doc(user).get()
        )
      );

      const messagePromises = [];
 
      // Iterate over all documents and add a message for every existing document
      customRoomUsersSnap.forEach((userSnap) => {
        if (userSnap.exists) {
          const message = {
            data: {
              title: `New message from ${capitalizeFirstLetter(
                snapshot.data().room
              )}`,
              body:
                snapshot.data().type == "m.text"
                  ? "Sent a new message"
                  : snapshot.data().type == "m.start"
                  ? "Invited you for a call"
                  : snapshot.data().type == "m.image"
                  ? "Sent an image"
                  : "Sent a new message",
            },
            tokens: userSnap.data()["Device_token"],
            android: {
              priority: "high",
            },
            priority: "high",
          };

          messagePromises.push(admin.messaging().sendMulticast(message));
        } else {
          console.log(`User ${userSnap.ref.id} doc does not exist`);
        }
      });

      // Send all messages simultaneously.
      return Promise.all(messagePromises);
    } catch (error) {
      console.log(error);

      // return Promise<null> in case of an error
      return null;
    }
  });