Query with scanIndexForward=true and limit=1

I have a dynamo db table that looks something like this:

My main requirement is to fetch the oldest item in a partition (based on CreationTime). Essentially, should behave like a FIFO queue.

I'm using DynamoDbMapper to query this table using the partition key value, scanIndexForward=true and limit=1. Now when i try to get the first item in the results, it takes around 25 seconds (there were ~15,000 items in the partition). Here's the function used:

public Optional<MyDdbItem> getLastItemInPartition(@NonNull final String partitionKey) {
        final DynamoDBQueryExpression<MyDdbItem> queryExpression =
                new DynamoDBQueryExpression<MyDdbItem>()
                        .withHashKeyValues(MyDdbItem.builder()
                                .requestType(partitionKey).build())
                        // This is set true for ascending order of sort key (creationTime)
                        .withScanIndexForward(true)
                        // Fetch only one item
                        .withLimit(1);
        Optional<MyDdbItem> result = Optional.empty();
        try {
            final PaginatedQueryList<MyDdbItem> results =
                    getDynamoDBMapper().query(MyDdbItem.class, queryExpression);
            result = results.stream().findFirst();
        } catch (final Exception exception) {
            DDBExceptionHandler.handleDDBExceptions(exception);
        }
        return result;
    }

Things i verified:

1. The table metrics indicate that the query latency has never gone beyond 2 ms.
2. Performed a similar query from a python script, and it took around 500 ms.

Python script:

def get_last_item_in_partition(table_name, partition_key_value):
    query_params = {
    'TableName': table_name,
    'KeyConditionExpression': '#pk = :pkval',
    'ExpressionAttributeNames': {'#pk': 'RequestType'},
    'ExpressionAttributeValues': {':pkval': {'S': partition_key_value}},
    'ScanIndexForward': True,  
    'Limit': 1 
    } 

    try:
        response = dynamodb.query(**query_params)
        items = response['Items']
        if items:
            return items[0]
        else:
            return None
    except NoCredentialsError as e:
        print(f"Error: {e}")
        return None

Posting here to verify if the index-design accommodates my requirement.