For each loop slow perfomance

Why are you trying to serialize individual rows? You are performing a couple of thousand operations here when serializing the entire data set will only do one serialization operation. Change the code to serialize everything and you should see improvements.

At a guess, you're serializing each row to a string and concatenating it to a larger string - eg:

// DON'T DO THIS!!!
string result = "";
foreach (var row in table)
{
    result += SerializeToJson(row);
}

String concatenation in a loop is horrendously slow. For each row, you have to create a new string, copy the characters from the existing string, append the characters from the new string, and store the result in the variable.

Instead, you should use a StringBuilder[^], or stream the results to the output directly.

var sb = new StringBuilder();
foreach (var row in table)
{
    sb.Append(SerializeToJson(row));
}

string result = sb.ToString();

// Or:
using (Stream stream = File.Create("... some path ..."))
using (TextWriter writer = new StreamWriter(stream))
{
    foreach (var row in table)
    {
        writer.Write(SerializeToJson(row));
    }
}

// Or, using JSON.NET:
using (Stream stream = File.Create("... some path ..."))
using (TextWriter writer = new StreamWriter(stream))
using (JsonTextWriter jsonWriter = new JsonTextWriter(writer))
{
    JsonSerializer serializer = new JsonSerializer();
    foreach (var row in table)
    {
        serializer.Serialize(jsonWriter, row);
    }
}

How to concatenate multiple strings (C# Guide) | Microsoft Docs[^]