Lambda too slow to query DynamoDB with recursive promises - javascript, sharding

I'm trying to fetch the most recently created items in a dynamodb table. For that I'm using a pattern described by Alex Debrie in his dynamoddb book plus sharding. When a new item is created in the table it also feeds a GSI with a GSIPK that is made out of the item creation day plus a random shard number between 0 and 9. The SK would be the item unique ID

GSI1

• GSI1PK: truncated timestamp#[0-9]
• GSI1SK: item id

there can be few dozens of recently created items or thousands of items.

To fetch the most recent items I have three (3) parameters:

• Date: The current day
• Limit: total amount of items to fetch
• Days:number of days back to look for items

As suggested by Alex Debrie book the method to retrieve the items is a recursive function with promises.

The problem that I'm facing is that my lambda function is very slow.

in the scenario that there are not so many items created recently, the function has to go through all the days+shards one after another to fetch items. for example. If I want to fetch the last 100 items in the last 7 days. and there are less than 100 items spread across the shards. The function will go through 70 Queries (7 days x 10 shards) and it takes around 10 seconds to finish

On the contrary if I want to fetch 100 items in the last 7 days and hundreds of items were created recently, then it till take around a second to run.

• items are small. around 400 bytes each.
• I'm running an on-demand capacity dynamodb table
• Lambda is configured with memorySize: 1536MB
• Node.js 16.x
• Any ideas how can make this run faster ?

const getQueryParams = (createdAt, shard, limit) => {
    const params = {
        TableName : "table",
        IndexName: 'GSI1',
        KeyConditionExpression: "#gsi1pk = :gsi1pk",
        ExpressionAttributeNames: {
            "#gsi1pk": 'GSI1PK' 
        },
        ExpressionAttributeValues: {
            ":gsi1pk": `${truncateTimestamp(timestamp).toISOString()}#${shard}` //e.g 2023-02-09T00:00:00.000Z#8
        },
        ScanIndexForward: false,
        Limit: limit
    };
    return params;

}

const getItems = async => {

    const items = []
    const number_of_days = 3;

    const getLatestItems = async ({ createdAt = new Date(), limit = 100, days = 0, shard = 0 }) => {
        
        const query_params = getQueryParams(createdAt, shard, limit);

        let max_items_to_fetch = limit;

        return dynamoDb.query(query_params).then(
            (data) => {
                // process data.
                if (data.Items) {
                    data.Items.forEach((item) => {
                        if (items.length < limit) {
                            items.push(item);
                        }
                    })
                    max_items_to_fetch = limit - data.Items.length;
                }    
                
                if (items.length >= limit) {
                    return items;
                }

                if (shard < 9) {
                    let params = {
                        createdAt: new Date(createdAt.setDate(createdAt.getDate())),
                        limit: max_items_to_fetch,
                        days: days,
                        shard: shard + 1,
                    }
                    return getLatestItems(params);
                
                } else if (days < number_of_days) {
                    let params = {
                        createdAt: new Date(createdAt.setDate(createdAt.getDate() - 1)),
                        limit: max_items_to_fetch,
                        days: days + 1,
                        shard: 0,
                    }
                    return getLatestItems(params);
        
                }
                return items;
            },
            (error) => {
                    throw new Error('Error getting all recent itmems')
            }
        );
    }

    return getLatestItems({});
};


export const main = async (event) => {
    const start = Date.now();
    const itemPromises = getItems();
    const res = await Promise.all([itemPromises]);
    const end = Date.now();
    console.log(`Execution time: ${end - start} ms`);
};