C# .NET Core S3 Object Listing Performance

Function.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

using Amazon.Lambda.Core;
using Amazon.S3;
using Amazon.S3.Model;

// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

namespace temp_di_1514_3
{
    /// 
    /// TODO BW: Seems like this is retaining state somehow between invocations.
    /// I need to Extract Class and create a new one each time, instead of having state variables
    /// in the exported Function class.
    public class Function
    {
        string S3_MERGE_BUCKET = "prod-transactions-merge.upside-services.com";
        string S3_MERGE_PENDING_PREFIX = "pending";

        int DEFAULT_MAX_KEYS = 10000;

        IAmazonS3 _s3Client;

        string _continuationToken;

        int _keyCount;

        int _maxKeys;

        DateTime _oldestTimestamp = DateTime.MaxValue;

        /// <summary>
        /// A simple function that takes a string and does a ToUpper
        /// </summary>
        /// <param name="input"></param>
        /// <param name="context"></param>
        /// <returns></returns>
        public async Task<string> FunctionHandler(string input, ILambdaContext context)
        {
            // Dirty hack.
            _continuationToken = null;
            _keyCount = 0;

            _maxKeys = string.IsNullOrWhiteSpace(input) ? DEFAULT_MAX_KEYS : int.Parse(input);

            _s3Client = new AmazonS3Client();

            var pendingPage = GetNextPage();
            ListObjectsV2Response currentPage = null;

            do 
            {
                currentPage = await pendingPage;
                if(currentPage.IsTruncated && _keyCount < _maxKeys)     // TODO BW: DRY.
                {
                    pendingPage = GetNextPage();
                }   

                FindOldest(currentPage.S3Objects);

                System.Console.WriteLine($"{_keyCount}");
            }
            while(currentPage.IsTruncated && _keyCount < _maxKeys);    // TODO BW: DRY.
            
            System.Console.WriteLine($"Oldest LastModifed: {_oldestTimestamp.ToString("o")}");

            return input?.ToUpper();
        }

        /**
        TODO BW: This would be better if implemented as an async iterator continuation.
        */
        async Task<ListObjectsV2Response> GetNextPage() 
        {
            var page = await _s3Client.ListObjectsV2Async(new ListObjectsV2Request 
            { 
                BucketName = S3_MERGE_BUCKET,
                Prefix = S3_MERGE_PENDING_PREFIX,
                ContinuationToken = _continuationToken
            });

            _keyCount += page.KeyCount;
            _continuationToken = page.IsTruncated ? page.NextContinuationToken : null;

            return page;
        }

        void FindOldest(IEnumerable<S3Object> s3Objects) 
        {
            foreach(var objectInfo in s3Objects)
            {
                if(objectInfo.LastModified < _oldestTimestamp) 
                {
                    _oldestTimestamp = objectInfo.LastModified;
                }
            }
        }
    }
}

C# performance with S3 file listing isn't bad.
10k files: 1.4s
188,368 files: 25.9s