Boost 5.x -> 7.x migartion performance for Gbs of data (C# Nest)

I need to migrate existing ES data from version 5.2.2 to 7.6.0.
I do that by reindexing index per index to V6 and then bulk inserting index per index to a new created index and then delete the source index.
I cannot use reindexing for that because I want to modfiy the data on the fly and I don't want to use ES specific update script.

This is my code for reindexing to v6:

            var result = HttpClient.PostAsync(ReIndexUrl, new StringContent(JsonConvert.SerializeObject(reindexModel.Value), Encoding.UTF8, "application/json")).Result;
            var response = result.Content.ReadAsStringAsync().Result;
            var responseObj = JsonConvert.DeserializeObject<ReindexResponse>(response);
            return responseObj.Total;

Note that reindexModel.Value is like this:

{ source = new { index = "sourceIndex" , type = "myDocType", size = 5000}, dest = new { index = targetIndex, type = "_doc" } 

and ReindexResponse.Total is a long.

This is my code for reindexing to v7:

var dataPerIndex = data.Select(
                    item => new BulkIndexOperation<T>(item)
                                    Index = indexName

                var allBulksRequest = new BulkRequest
                                              Operations = new BulkOperationsCollection<IBulkOperation>(dataPerIndex),
                                              Refresh = Refresh.False

                if (allBulksRequest.Operations.Any())
                    var bulkResponse = elasticClient.Bulk(allBulksRequest);
                    if (bulkResponse.Errors || bulkResponse.ItemsWithErrors.Any())
                        throw new Exception($"BulkInsert for index: {indexName} failed with errors: {bulkResponse.DebugInformation}");

Note that size of data is streamed from source index and is chunked to 5000 just like I do set size = 5000 when reindexing to v6.

This works fine but takes quite a while!
In my example with 160 indices with roughly 2GB size total, the process took about 45 minutes!
Not to imaging what happens in case of having like 200GB of data.

Interesting: Increasing value 5000 to like 10000 (which is max value without tweaking) did not improve performance at all.
Any ideas on that?
How do you solve this issue?

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