Cloud Firestore taking too much time fetch 100K records in my Flutter App

I am creating a Store Ranking Algorithm, in which I have to fetch products from the user list and compare them with stores. Earlier my solution was taking n+1 query problem. So to optimize that I am trying to download as much data as possible at the mobile app. This is not a good approach but this is what I see as a solution.

I have 100K records in the products collection. 90 Records in store so far (will increase).

The Product has straight forward schema with name, image, measure, category, branch, storeRef etc. fields.

And user_product_list has quantity and product_ref fields.

Here is my full code.

Main evil of the loading time is storeProductsSnapshot. I am just trying to download data of store for offline algorithm.

  Future<void> rankStoresByPriceTotal2({double radius = 6}) async {
    
    final listId = await prefs.then((prefs) => prefs.getUserListId());

    try {
      final List<Stores> nearbyStores = await getNearbyStores(
        radius: radius,
        firestore: fireStore,
        userLocation: GeoPoint(20.68016662, -103.3822084),
      );

      // Creating StoreRef List for better batched read products
      final List<DocumentReference> storeRefs = nearbyStores
          .map((store) =>
          fireStore.collection(StoreConstant.storeCollection).doc(store.id))
          .toList();

      // Fetch all store products that meet the conditions
      // Main Evil who takes too much time.
      final QuerySnapshot storeProductsSnapshot = await fireStore
          .collection('products')
          .where('is_exist', isEqualTo: true)
          .where('storeRef', whereIn: storeRefs)
          .get();

      // Create a list of Product objects
      final storeProducts = storeProductsSnapshot.docs.map((productSnapshot) {
        final ref = productSnapshot.reference;

        return Product(
          productId: productSnapshot['product_id'].toString(),
          productRef: ref.id.toString(),
          isExist: productSnapshot['is_exist'] as bool,
          name: productSnapshot['name'].toString(),
          pImage: productSnapshot['pImage'].toString(),
          storeRef: productSnapshot['storeRef'].path.toString(),
          price: productSnapshot['price'].toString(),
        );
      }).toList();


      // Fetch the user's product list from the user_product_list subcollection
      QuerySnapshot productListSnapshot = await fireStore
          .collection('mylist')
          .doc(listId)
          .collection('user_product_list')
          .get();

      // Build a list of product references to fetch in a batched read
      final List<DocumentReference> productRefs = productListSnapshot.docs
          .map((productSnapshot) =>
      productSnapshot['product_ref'] as DocumentReference)
          .toList();

      // Fetch the products in a batched read
      final List<Product> userProducts = await Future.wait(
        productRefs.map(
              (ref) async {
            final productSnapshot = await ref.get();

            return Product(
              productId: productSnapshot['product_id'].toString(),
              productRef: ref.id.toString(),
              isExist: productSnapshot['is_exist'] as bool,
              name: productSnapshot['name'].toString(),
              pImage: productSnapshot['pImage'].toString(),
              storeRef: productSnapshot['storeRef'].path.toString(),
              price: productSnapshot['price'].toString(),
            );
          },
        ).toList(),
      );

      // Some Complex Algorithm to Find Best Stores using user products.
      final Map<DocumentReference, double> storeTotal = {};

      for (var storeRef in storeRefs) {
        for (var i = 0; i < productListSnapshot.size; i++) {
          final productSnapshot = productListSnapshot.docs[i];
          final product = userProducts[i];
          final price = double.parse(product.minPrice);
          final quantity = productSnapshot['quantity'];
          final productId = product.productId;


          if (product.isExist && product.storeRef == storeRef.path) {
            storeTotal[storeRef] =
                (storeTotal[storeRef] ?? 0) + (quantity * price);

            continue;
          }

          // Some product matching formula to find similar product
          final Product? productInStore = _getSimilarProductFromList(
            storeRef: storeRef.path,
            product: product,
            storeProducts: storeProducts,
          );
          if (productInStore == null) {

            // Some light operations

          } else if (productInStore.productId == productId) {
            /// Matching product price
            final price = double.parse(productInStore.minPrice);

            storeTotal[storeRef] =
                (storeTotal[storeRef] ?? 0) + (quantity * price);

            // Some light operations
            
          } else {
            /// similar product price
            final price = double.parse(productInStore.minPrice);

            storeTotal[storeRef] =
                (storeTotal[storeRef] ?? 0) + (quantity * price);

            // Some light operations
            
          }
        }
      }

      // Sort the found stores with the products.  
      
    } catch (e) {
      print('Error ranking stores: $e');

      
    }
  }