Javascript Firestore获取子集合的速度很慢

我有收藏和它们的子收藏。我在它们上面循环并收集数据。它工作正常，但速度太慢了。有人能建议改进性能问题吗

static async getSubCategories(category_id) {
    const db = Firebase.firestore(),
        subCategories = [];
        activeRef = await db.collection("sub_categories").where("category_id", "==", category_id).orderBy("id").get();
    for (let doc of activeRef.docs) {
        const subCategory = doc.data();
        if (_.isObject(subCategory)) {
            const subRef = await doc.ref.collection('sub_sub_categories').orderBy("id").get(),
                subSubCategories = [];
            for (let subDoc of subRef.docs) {
                const subSubCategory = subDoc.data();
                if (_.isObject(subCategory)) {
                    subSubCategories.push(subSubCategory);
                    // If SubCategory has image, download storage uri
                    if (subSubCategory.image_storage_uri) {
                        const imageRef = Firebase.storage().refFromURL(subSubCategory.image_storage_uri),
                            imageUri = await imageRef.getDownloadURL();
                        subSubCategory.image_uri = imageUri;
                    }
                }
            }
            if (subSubCategories.length > 0)
                subCategory.sub_sub_categories = subSubCategories;
            subCategories.push(subCategory);
        }
    }
    return subCategories;
}

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你说的“太慢了”是什么意思？有多少结果返回您的查询？我有大约30个集合，每个集合有大约6-8个子集合文档。那么总共有多少文档返回您的查询？我只是担心我的代码可能是错误的……总共收集了大约250个。